WASHINGTON -- The White House said Thursday that it believes the Syrian government has used chemical weapons in its civil war, an assessment that could test President Obama's repeated warnings that such an attack could precipitate American intervention in Syria.
The White House, in a letter to Congressional leaders, said the nation's intelligence agencies assessed "with varying degrees of confidence" that the government of President Bashar al-Assad had used the chemical agent sarin on a small scale.
But it said more conclusive evidence was needed before Mr. Obama would take action, referring obliquely to both the Bush administration's use of faulty intelligence in the march to war in Iraq and the ramifications of any decision to enter another conflict in the Middle East.
Senator Dianne Feinstein, Democrat of California, who is chairwoman of the Senate Intelligence Committee, said the agencies actually expressed more certainty about the use of these weapons than the White House indicated in its letter. She said Thursday that they voiced medium to high confidence in their assessment, which officials said was based on the testing of soil samples and blood drawn from people who had been wounded.
American officials said the attacks, which occurred last month in a village near Aleppo and in the outskirts of Damascus, had not been definitively connected to Mr. Assad. The White House said the "chain of custody" of the weapons was not clear, raising questions about whether the attacks were deliberate or accidental.
"Given the stakes involved, and what we have learned from our own recent experience, intelligence assessments alone are not sufficient," the White House said in the letter, which was signed by its legislative director, Miguel E. Rodriguez. "Only credible and corroborated facts that provide us with some degree of certainty will guide our decision-making."
That meticulously legal language did not disguise a thorny political and foreign policy problem for Mr. Obama: he has long resisted the calls to arm the Syrian rebels and has expressed deep doubts about the wisdom of intervening in an Arab nation so riven with sectarian strife, although he has also issued pointed warnings to Syria.
In a statement last summer, Mr. Obama did not offer a technical definition of his "red line" for taking action, but said it was when "we start seeing a whole bunch of weapons moving around or being utilized." In Jerusalem last month, he said proof that Syria had used such weapons would be a "game changer" for American involvement.
The Pentagon, administration officials said, has prepared the president a menu of options that include commando raids that would secure chemical weapons stockpiles and strikes on Syrian planes from American ships in the Mediterranean. Last year, the United States secretly sent a 150-member task force to Jordan to help deal with the possibility that Syria would lose control of its stockpiles. Mr. Obama could also provide more robust aid to the rebels, including weapons.
White House officials gave no indication of what Mr. Obama might do, except to say that any American action would be taken in concert with its allies.
While lawmakers from both parties swiftly declared that the president's red line had been breached, they differed on what he should do about it.
"The political reality is that he put himself in that position that if the 'red line' is crossed -- he made it very clear -- it would change his behavior," Senator John McCain, Republican of Arizona, said. The intelligence "is a compelling argument for the president to take the measures that a lot of us have been arguing for all along," he said.
The timing of the White House disclosure also suggested the pressures it is facing. It came the same day that the British government said that it had "limited but persuasive" evidence of the use of chemical weapons, and two days after an Israeli military intelligence official asserted that Syria had repeatedly used chemical weapons.
In a letter to the United Nations secretary general, Ban Ki-moon, several weeks ago calling for a United Nations investigation, Britain laid out evidence of the attacks in Aleppo and near Damascus as well as an earlier one in Homs.
The letter, a copy of which was obtained by The New York Times, reported that dozens of victims were treated at hospitals for shortness of breath, convulsions and dilation of the pupils, common symptoms of exposure to chemical warfare agents. Doctors reported eye irritation and fatigue after close exposure to the patients.
Citing its links to contacts in the Syrian opposition, Britain said there were reports of 15 deaths in the suburban Damascus attack and up to 10 in Aleppo, where the government and rebels have each accused the other of using chemical weapons.
"Fortunately the deaths have not been high," Senator Feinstein said, "but there have been deaths."
The United States has also pushed for a United Nations investigation, but it made clear on Thursday that it has collected enough evidence on its own and with Britain and other countries to make its assessment. An official said the United States was also suspicious about the attack in Homs.
While several officials said the intelligence agencies expressed medium to high confidence about its overall assessment, two intelligence officials noted that there were components of the assessment about which the agencies were less certain. They did not offer details.
Administration officials had begun the week casting doubt on the claims made by the Israeli official, Brig. Gen. Itai Brun, about chemical weapons. "Suspicions are one thing; evidence is another," Defense Secretary Chuck Hagel said Wednesday on a visit to Egypt.
But by then, a senior administration official said, the intelligence agencies had already become more confident of their assessment, after several weeks of examining the evidence. With Secretary of State John Kerry scheduled to brief senators on Syria on Thursday, the White House decided on Wednesday evening to get ahead of that meeting.
The administration's disclosure came in a response to Mr. McCain, a senior member of the Senate Armed Services Committee, and the committee's chairman, Senator Carl Levin of Michigan, who wrote to the White House asking whether Mr. Assad or his supporters had used chemical weapons during the two-year-long war.
"Given the fact that we have been developing additional information within our intelligence community," a White House official said to reporters, "we felt it was the right and prudent thing to do to respond in an unclassified form to this letter."
Lawmakers generally welcomed the White House's disclosure, though some suggested that the administration was still inclined to play down the implications of the assessment.
"It is important that we read the intelligence as it is laid out, not as we would like it to be," said Representative Mike Rogers, Republican of Michigan and the chairman of the House Intelligence Committee.
Michael R. Gordon contributed reporting from Washington; Thom Shanker from Abu Dhabi, United Arab Emirates; and David E. Sanger from Jerusalem.
This article originally appeared in The New York Times.