VIENNA — Inspectors have located radioactive traces at an Iranian underground bunker, the U.N. atomic agency said Friday – a finding that could mean Iran has moved closer to reaching the uranium threshold needed to arm nuclear missiles.

In a report obtained by The Associated Press, the International Atomic Energy Agency said it was asking Tehran for a full explanation. But the report was careful to avoid any suggestion that Iran was intentionally increasing the level of its uranium enrichment, noting that Tehran said a technical glitch was responsible.

Analysts as well as diplomats who had told the AP of the existence of the traces before publication of the confidential report also said the higher-enriched material could have been a mishap involving centrifuges over-performing as technicians adjusted their output rather than a dangerous step toward building a bomb.

Still, the finding was bound to resonate among the 35 IAEA board members for whom the report was prepared, among them the six world powers that had just concluded talks with Iran on its enrichment activities. The negotiations in Baghdad left the two sides still far apart over how to oversee Tehran's atomic program but resolved to keep dialogue going next month in Moscow as an alternative to possible military action.

The report also expressed in opaque terms what diplomats accredited to the IAEA first started mentioning more than a month ago: The agency fears a massive cleanup under way at buildings at the Parchin military complex southeast of Tehran. The site is suspected of housing a pressure chamber and related equipment used to test ways of detonating a nuclear charge.

"Based on satellite imagery at this location ... the buildings of interest to the agency are now subject to extensive activities that could hamper the agency's ability to undertake effective verification" at the site, said the report. Separately, a senior international official familiar with the issue said satellite photos showed trucks at the site and streams of liquid suggesting the interiors were being hosed down to wash away evidence of possible nuclear-related work.

The report said that over the past five months the agency had "obtained more information" buttressing its suspicions about the Parchin site, which it has asked repeatedly to visit, only to be turned down by Iran.

On enrichment, the six world powers trying to engage Iran are already concerned about its output of uranium at the 20 percent level because material that high can be turned into weapons-grade much more quickly than its main, low-enriched stockpile.

The higher the enrichment, the easier it becomes to re-enrich uranium to warhead quality at 90 percent. As a result, the finding of traces at 27 percent at the Fordo enrichment plant in central Iran sparked international interest.

Iran denies any plans to possess nuclear weapons but has for years declined offers of reactor fuel from abroad, including more recent inducements of 20-percent material if it stops producing at that level. The Islamic Republic says it wants to continue producing 20 percent uranium to fuel its research reactor and for medical purposes.

But its refusal to accept foreign offers has increased fears it may want to turn its enrichment activities toward producing such arms. The concerns have been fed by IAEA suspicions that Iran has experimented on components of an atomic arms program – suspicions Tehran also denies.

The report cited a May 9 letter from Iranian officials suggesting any enrichment at 27 percent was inadvertent. The letter said the particles were produced "above the target value" and could have been for "technical reasons beyond the operator's control."

David Albright, whose Washington-based Institute for Science and International Security looks for signs of proliferation, said a new configuration at Fordo means it tends to "overshoot 20 percent" at the start.

"Nonetheless, embarrassing for Iran," he wrote in an email to the AP.

Mark Fitzpatrick, director of the Non-Proliferation and Disarmament Program of the International Institute for Strategic Studies, described such a surge as a "naturally occurring development," adding that "it's very usual" to go above envisaged enrichment levels at startups of centrifuges.

In Washington, State Department spokeswoman Victoria Nuland said "there are a number of possible explanations for this, including the one that the Iranians have provided. But we are going to depend on the IAEA to get to the bottom of it."

Others were more skeptical.

"It's not surprising because they have the technology," a senior Israeli defense official said.

"Iran doesn't intend to stop its nuclear weapon program, and the fact that they are at 27 percent shows the Iranian intentions," said the official, who spoke on condition of anonymity because he is not allowed to speak with the media.

International concerns have increased since Iran started higher enrichment at Fordo, which is carved into a mountain to make it impervious to attack. Israel and the United States have not ruled out using force as a last option if diplomacy fails to curb the Islamic Republic's nuclear program.

Iran already has about 700 centrifuges churning out 20-percent enriched uranium at Fordo. The report noted that although Iran has set up about 350 more centrifuges since late last year at the site, these machines are not enriching.

While the reason for that could be purely technical, it could also be a signal from Tehran that it is waiting for progress in the negotiations.

The latest attempt to persuade Iran to compromise ended inconclusively Thursday after two days of talks in Baghdad. The six world powers – the United States, Russia, China, Britain, France and Germany – failed to persuade Tehran to freeze its 20 percent enrichment.

Iran, for its part, failed to persuade the West to scale back on recently toughened sanctions, which have targeted Iran's critical oil exports and have effectively blackballed the country from international banking networks. The 27-nation European Union is set to ban all Iranian fuel imports on July 1, shutting the door on about 18 percent of Iran's market.

But the IAEA report did detail some progress in separate talks between the U.N. nuclear agency and Iran that the agency hopes will re-launch a long-stalled probe into the suspicions that Tehran has worked on nuclear-weapons related experiments.

The report said that since 2002, the IAEA "has become increasingly concerned about the possible existence in Iran of undisclosed nuclear-related activities involving military-related organizations, including activities related to the development of a nuclear payload."

Without more openness on the part of Iran, said the report, the IAEA "cannot conclude that all nuclear material in Iran is in peaceful activities."

___

AP writer Ian Deitch in Jerusalem contributed this report.

Related on HuffPost:

Loading Slideshow...
  • <em>In this Saturday, Nov. 12, 2011 photo, Iranian women and a man weave carpet in a workshop in Qom, 78 miles (125 kilometers) south of the capital Tehran, Iran. (AP Photo/Vahid Salemi)</em><br><br> Initial sanctions were imposed after Iranian students stormed the U.S. embassy and took diplomats hostage in 1979. Iranian products cannot be imported into the United States apart from small gifts, information material, food and some carpets.

  • <em>Former President Bill Clinton addresses the audience during the opening night dinner of the World Summit of Nobel Peace Laureates at the Field Museum Monday, April 23, 2012, in Chicago. (AP Photo/Charles Rex Arbogast)</em><br><br> In 1995, President Bill Clinton issued executive orders preventing U.S. companies from investing in Iranian oil and gas and trading with Iran. The same year, Congress passed a law imposing sanctions on foreign companies investing more than $20 million a year in Iran's energy sector.

  • <em>In this Thursday, Oct. 30, 2008 file photo, an Iranian money changer holds currency with Ayatollah Ruhollah Khomeini's image in Tehran, Iran. (AP Photo/Hasan Sarbakhshian, File)</em><br><br> In October 2007, Washington imposed sanctions on three Iranian banks and branded the Islamic Revolutionary Guards Corps a proliferater of weapons of mass destruction. The Treasury has since added numerous other Iranian banks to its blacklist. The Treasury has identified about 20 petroleum and petrochemical companies as being under Iranian government control, an action that put them off-limits to U.S. businesses under the trade embargo.

  • <em>This photo shows a branch of Iranian Bank Tejarat in Tehran on January 24, 2012 upon which the US Treasury announced sanctions claiming all of the Islamic Republic's major state-owned banks have now been subjected to punitive measures. (ATTA KENARE/AFP/Getty Images)</em><br><br> Congress approved tough new unilateral sanctions on June 24, 2010, aimed at squeezing Iran's energy and banking sectors. The new law imposed penalties on companies that supply Iran with refined petroleum products worth more than $5 million a year. It also effectively deprived foreign banks of access to the U.S. financial system if they did business with Iranian banks or the Revolutionary Guards.

  • <em>Oil workers gather by an oil well operated by Venezuela's state-owned oil company PDVSA in Morichal, Venezuela, on July 28, 2011. (RAMON SAHMKOW/AFP/Getty Images)</em><br><br> In May 2011, the United States announced new sanctions on Venezuela's state oil company, PDVSA, and six other smaller oil and shipping firms for trading with Iran in violation of the U.S. ban, prompting fury from Hugo Chavez's government.

  • June 2011

    <em>Members of Iran's paramilitary Basij militia parade in front of the former US embassy in Tehran on November 25, 2011 to mark the national Basij week. (ATTA KENARE/AFP/Getty Images)</em><br><br> On June 11, it announced new sanctions on the Revolutionary Guards, the Basij Resistance Force, and Iran's law enforcement forces. The sanctions froze any of the targets' assets under U.S. jurisdiction and barred U.S. persons and institutions from dealing with them.

  • <em>Secretary of State Hillary Clinton and Treasury Secretary Timothy Geithner (L) announce new sanctions against Iran at the State Department on November 21, 2011 in Washington, DC. (Photo by Mark Wilson/Getty Images)</em><br><br> On Nov. 21 the United States named Iran as an area of "primary money-laundering concern", a step designed to dissuade non-U.S. banks from dealing with it. The United States also blacklisted 11 entities suspected of aiding its nuclear programmes and expanded sanctions to target companies that aid its oil and petrochemical industries.

  • <em>US President Barack Obama (C) greets guests after speaking on nuclear security, touching on subjects from terrorism to Iran and North Korea, during a visit to Hankuk University of Foreign Studies in Seoul on March 26, 2012. (SAUL LOEB/AFP/Getty Images)</em><br><br> On Dec. 31, 2011, President Barack Obama signed into law a defense funding bill that imposed sanctions on financial institutions dealing with Iran's central bank, which is the main conduit for oil revenues. Sanctioned institutions would be frozen out of the U.S. financial markets.

  • <em>Hostesses stand in front of the construction site of the Saudi Basic Industries Corp. (SABIC) Technology Centre and Greater China Headquarters in Shanghai, China Friday, April 6, 2012. (AP Photo/Eugene Hoshiko)</em><br><br> On Jan. 13, 2012 the United States extended sanctions to Chinese state-run energy trader Zhuhai Zhenrong Corp, which it said was Iran's largest supplier of refined petroleum products. It also imposed sanctions on Singapore's Kuo Oil Pte Ltd and United Arab Emirates-based FAL Oil Company Ltd.

  • <em>Members of Iranian Revolutionary Guards attend a ceremony at the mausoleum of the late revolutionary founder Ayatollah Khomeini, commemorating 33rd anniversary of his return from exile after 14 years, and the 1979 Islamic Revolution which toppled pro-US Shah Mohammad Reza Pahlavi, just outside Tehran, Iran, Wednesday, Feb. 1, 2012. (AP Photo/Vahid Salemi)</em><br><br> The United States exempted Japan and 10 European Union nations from financial sanctions on March 20 because they had significantly cut purchases of Iranian oil, but Iran's top customers China and India remain at risk of such steps. On March 28 the Treasury set additional sanctions against Iranian engineering firms with ties to the Revolutionary Guards, as well as individuals and shipping companies with ties to the Islamic Republic of Iran Shipping Lines (IRISL).

  • <em>In this Sept. 27, 2000 file photo, an Iranian oil worker repairs a pipe at an oil refinery in Tehran. (AP Photo/Vahid Salemi, File)</em><br><br> On May 21 the Senate approved the latest tightening of sanctions on Iran's oil trade.

  • <em>In this Jan. 19, 2012 file photo, fishing boats are seen in front of oil tankers on the Persian Gulf waters, south of the Strait of Hormuz, offshore the town of Ras Al Khaimah in United Arab Emirates. (AP Photo/Kamran Jebreili)</em><br><br> On Aug. 12, 2010 the EU banned the creation of joint ventures with enterprises in Iran engaged in the oil and natural gas industries. Member states must prohibit the provision of insurance and re-insurance to the government of Iran. The import and export of arms and equipment that could contribute to uranium enrichment, or have a "dual use", is banned. The sanctions forbid the sale, supply or transfer of energy equipment and technology used by Iran for exploration and production or for refining or liquefying natural gas. The EU expects the effects of the sanctions to increase over time as existing parts wear out.

  • <em>EU foreign policy chief Catherine Ashton speaks during a media conference after a meeting of EU foreign ministers at the EU Council building in Brussels on Monday, Jan. 23, 2012. (AP Photo/Virginia Mayo)</em><br><br> In May 2011, EU foreign ministers added 100 new entities to a list of companies and people affected, including those owned or controlled by IRISL. Last October, the EU imposed sanctions on 29 people, extending the list targeting individuals associated with human rights violations to 61. On Dec. 1, the EU added 180 Iranians and entities to a sanctions blacklist that imposes asset freezes and travel bans on those involved in the nuclear program.

  • <em>In this March 13, 2008 file photo, gold coins and bars are shown at California Numismatic Investments in Inglewood, Calif. (AP Photo/Nick Ut, File)</em><br><br> On Jan. 23, 2012 the EU placed an immediate ban on all new contracts to import, purchase or transport Iranian crude oil and petroleum products. EU countries with existing contracts to buy oil and petroleum products were allowed to honor them until July 1. The EU also agreed to freeze the assets of Iran's central bank and ban trade in gold and other precious metals with the bank and state bodies.

  • <em>British Ambassador to the UN Mark Lyall Grant (C) speaks during a vote on broader military and financial sanctions on Iran over its suspect nuclear program during a UN Security Council at the UN headquarters June 9, 2010 in New York. (EMMANUEL DUNAND/AFP/Getty Images)</em><br><br> The Security Council has imposed four sets of sanctions on Iran, in December 2006, March 2007, March 2008 and June 2010. The first covered sensitive nuclear materials and froze the assets of Iranian individuals and companies linked with the nuclear program.

  • <em>A military truck carries a Sejil rocket as it is paraded during the annual Army Day military parade in Tehran on April 17, 2012. (ATTA KENARE/AFP/Getty Images)</em><br><br> The second included new arms and financial sanctions. It extended an asset freeze to 28 more groups, companies and individuals engaged in or supporting sensitive nuclear work or the development of ballistic missiles.

  • <em>Iranian President Mahmoud Ahmadinejad adjusts his goggles as he tours an exhibition on laser technology in Tehran on February 7, 2010. (ATTA KENARE/AFP/Getty Images)</em><br><br> The third, in 2008, increased travel and financial curbs on individuals and companies. It expanded a partial ban on trade in items with both civilian and military uses to cover sales of all such technology to Iran.

  • <em>A street money exchanger, puts US dollars in a plastic bag, in Ferdowsi St. in downtown Tehran, Iran, Wednesday, Dec. 21, 2011. (AP Photo/Vahid Salemi)</em><br><br> A Security Council resolution passed on June 9, 2010, called for measures against new Iranian banks abroad if a connection to the nuclear or missile programmes was suspected.

  • <em>Soldiers of the Iranian Revolutionary Guard walk past a satirical drawing of Statue of Liberty on the wall of the former US Embassy in Tehran, Iran, Friday, Nov. 25, 2011. (AP Photo/Vahid Salemi)</em><br><br> It expanded a U.N. arms embargo against Tehran and blacklisted three firms controlled by IRISL and 15 belonging to the Revolutionary Guards. The resolution called for the setting up of a cargo inspection regime.