SEOUL, South Korea — Just two months ago, North Korea blew up the cooling tower at its main nuclear reactor, a dramatic act meant to show the world it was committed to abandoning its atomic weapons ambitions.
That, coupled with a long-delayed account of its nuclear activities, fostered optimism and led last month to the first meeting between the foreign ministers of North Korea and the United States in four years.
But on Tuesday the communist nation said it had stopped disabling its Yongbyon nuclear complex on Aug. 14 and will consider restoring the plutonium-producing facility.
North Korea squarely blamed the United States for its decision, claiming Washington failed to keep its end of the deal.
"The U.S. postponed the process of delisting the (North) as a 'state sponsor of terrorism,'" the Foreign Ministry said in a statement carried by the state news agency. "Now that the U.S. breached the agreed points, the (North) is compelled to take" countermeasures, it said.
Most ominously, the North said it would "consider soon a step to restore" the nuclear facilities, though it provided no details.
Washington reacted calmly.
Secretary of State Condoleezza Rice said North Korea "still has obligations," adding that discussions were continuing. "I think we will just see where we will come out in a few weeks," she said during a visit to Ramallah, in the Palestinians' West Bank territory.
Removing North Korea from the list of state sponsors of terrorism was one of the key concessions the U.S. offered in exchange for North Korea shutting down and disabling the reactor.
In late June, the U.S. said it would remove the North from the list of state sponsors of terrorism after it turned in a long-awaited account of its nuclear programs and blew up the reactor's cooling tower.
The two sides have since been negotiating how to verify the nuclear declaration. Washington has been adamant that it will remove the North from the terror list only after the country agrees to a verification plan.
North Korea began disabling the nuclear facility last November, but slowed the work over the dispute with Washington over verification.
Sudden turnabouts by North Korea are not unusual. The United States, along with China, Japan, Russia and South Korea, have been working for five years to achieve North Korea's denuclearization.
Though that process has been characterized by many obstacles, most notably North Korea's underground detonation of a nuclear device in October 2006, Tuesday's announcement marks the most serious recent challenge.
"I think this represents the biggest crisis to the denuclearization process since the Feb. 13 agreement," said Yang Moo-jin, a professor at the University of North Korean Studies in Seoul, referring to a landmark disarmament-for-aid deal reached last year.
"North Korea hurled a strong message at the United States," Yang said.
The White House said the U.S. was sticking to its position.
"We've informed North Korea that we will take action to rescind its designation as a state sponsor of terrorism when it fulfills its commitment regarding verification," spokeswoman Dana Perino said, citing the "principle of 'action for action'" _ a favorite North Korean phrase.
The North's state media have issued a series of commentaries blasting the U.S., and the Foreign Ministry last week threatened that the country would bolster its "war deterrent" _ a euphemism for its nuclear arms programs.
Last week, the official Korean Central News Agency also lashed out at President Bush, accusing him of blocking progress at the nuclear talks by raising the issue of human rights in the North. During a trip to Seoul earlier this month, Bush publicly brought up the North's human rights record, the kind of criticism that has often angered its communist government.
South Korean and Japanese officials lamented the North's move.
"It's regrettable that this announcement came at a time when each side has been trying" to move the process forward, said Kim Sook, South Korea's chief nuclear envoy.
Japanese Foreign Ministry spokesman Kazuo Kodama said at a news conference that Tokyo viewed the North's action "with grave concern."
South Korean and U.S. officials have said that eight of the 11 disablement measures have been finished and that when the entire process is completed, it would take at least a year for the North to restart the facilities.
Robert Einhorn, a former assistant secretary of state for nonproliferation and now a senior adviser at the Center for Strategic & International Studies, doubts the North is willing to go that far.
"I think the threat to reverse the disablement process is a negotiating tactic," he wrote in an e-mail, saying the North Koreans have likely concluded the reactor is too old. "I don't think they have a genuine interest in taking the time, perhaps one year or longer, to restart it."
Associated Press writers Jae-soon Chang, Kwang-tae Kim and Hyung-jin Kim in Seoul, Shino Yuasa in Tokyo, Matthew Lee in Ramallah and Terence Hunt in Washington contributed to this report.