WASHINGTON -- The intelligence linking Syrian President Bashar Assad or his inner circle to an alleged chemical weapons attack is no "slam dunk," with questions remaining about who actually controls some of Syria's chemical weapons stores and doubts about whether Assad himself ordered the strike, U.S. intelligence officials say.

President Barack Obama declared unequivocally Wednesday that the Syrian government was responsible, while laying the groundwork for an expected U.S. military strike.

"We have concluded that the Syrian government in fact carried these out," Obama said in an interview with "NewsHour" on PBS. "And if that's so, then there need to be international consequences."

However, multiple U.S. officials used the phrase "not a slam dunk" to describe the intelligence picture – a reference to then-CIA Director George Tenet's insistence in 2002 that U.S. intelligence showing Iraq had weapons of mass destruction was a "slam dunk" – intelligence that turned out to be wrong.

A report by the Office of the Director for National Intelligence outlining that evidence against Syria includes a few key caveats – including acknowledging that the U.S. intelligence community no longer has the certainty it did six months ago of where the regime's chemical weapons are stored, nor does it have proof Assad ordered chemical weapons use, according to two intelligence officials and two more U.S. officials.

The humanitarian group Doctors Without Borders has said an Aug. 21 rocket strike killed 355 people.

A three-page report released Thursday by the British government said there was "a limited but growing body of intelligence" blaming the Syrian government for the attacks. And though the British were not sure why Assad would have carried out such an attack, the report said there was "no credible intelligence" that the rebels had obtained or used chemical weapons.

Quizzed by lawmakers in Britain's House of Commons, Prime Minister David Cameron gave various descriptions for his level of certainty to Assad's responsibility, ranging from "beyond doubt" to being "as certain as possible."

"We have a regime that has used chemical weapons on 14 occasions, that is most likely responsible for this large-scale attack, that if nothing is done it will conclude that it can use these weapons again and again and on a larger scale and with impunity," he said.

Like the British report, the yet-to-be-released U.S. report assesses with "high confidence" that the Syrian government was responsible for the attacks that hit suburbs east and west of Damascus, filled with a chemical weapon, according to a senior U.S. official who read the report.

The official conceded there are caveats in the report and there is no proof saying Assad personally ordered the attack. There was no mention in the report of the possibility that a rogue element inside Assad's government or military could have been responsible, the senior official said.

All the officials spoke on condition of anonymity because they were not authorized to discuss the intelligence report publicly.

Relevant congressional committees were to be briefed on that evidence by teleconference call on Thursday, U.S. officials and congressional aides said.

Administration officials said Wednesday that neither the U.N. Security Council, which is deciding whether to weigh in, nor allies' concerns would affect their plans. But the complicated intelligence picture raises questions about the White House's full-steam-ahead approach to the Aug. 21 attack on a rebel-held Damascus suburb, with worries that the attack could be tied to al-Qaida-backed rebels later.

Intelligence officials say they could not pinpoint the exact locations of Assad's supplies of chemical weapons, and Assad could have moved them in recent days as the U.S. rhetoric increased. But that lack of certainty means a possible series of U.S. cruise missile strikes aimed at crippling Assad's military infrastructure could hit newly hidden supplies of chemical weapons, accidentally triggering a deadly chemical attack.

Over the past six months, with shifting front lines in the 2 1/2-year-old civil war and sketchy satellite and human intelligence coming out of Syria, U.S. and allied spies have lost track of who controls some of the country's chemical weapons supplies, according to the two intelligence officials and two other U.S. officials.

U.S. satellites have captured images of Syrian troops moving trucks into weapons storage areas and removing materials, but U.S. analysts have not been able to track what was moved or, in some cases, where it was relocated. They are also not certain that when they saw what looked like Assad's forces moving chemical supplies, those forces were able to remove everything before rebels took over an area where weapons had been stored.

In addition, an intercept of Syrian military officials discussing the strike was among low-level staff, with no direct evidence tying the attack back to an Assad insider or even a senior Syrian commander, the officials said.

So while Secretary of State John Kerry said Monday that it was "undeniable," a chemical weapons attack had occurred, and that it was carried out by the Syrian military, U.S. intelligence officials are not so certain that the suspected chemical attack was carried out on Assad's orders. Some have even talked about the possibility that rebels could have carried out the attack in a callous and calculated attempt to draw the West into the war. That suspicion was not included in the official intelligence report, according to the official who described the report.

Ideally, the White House would prefer more clarity on all those points in the intelligence provided to it.

The U.S. has devoted only a few hundred operatives, between intelligence officers and soldiers, to the Syrian mission, with CIA and Pentagon resources already stretched by the counterterrorism missions in Africa and the Arabian Peninsula, as well as the continuing missions in Afghanistan and Pakistan, officials said.

The quest for added intelligence to bolster the White House's case for a strike against Assad's military infrastructure was the issue that delayed the release of the U.S. intelligence community's report, which had been expected Tuesday.

The uncertainty calls into question the statements by Kerry and Vice President Joe Biden.

"We know that the Syrian regime maintains custody of these chemical weapons," Kerry said. "We know that the Syrian regime has the capacity to do this with rockets. We know that the regime has been determined to clear the opposition from those very places where the attacks took place."

The CIA, the Pentagon and the Office of the Director of National Intelligence declined to comment, and the White House did not respond to requests for comment.

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Associated Press writers Bradley Klapper, Julie Pace and Lara Jakes contributed to this report.

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