Although the White House said it could not confirm the circumstances in which victims were exposed to chemical weapons, it said it believed that the chemical agent sarin had been used. "We do believe," the letter said, "that any use of chemical weapons in Syria would very likely have originated with the Assad regime."
Britain, in a letter last month requesting a United Nations investigation, cited three episodes in which it suspected that chemical weapons had been used: in a village west of Aleppo and on the outskirts of Damascus, both on March 19, and in Homs on Dec. 24.
Secretary of State John Kerry, emerging from a Congressional hearing, said that the United States believed that chemical weapons had been used in two instances, though he did not offer details.
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Faced with mounting pressure to act against Syria—including a new assertion by an Israeli military intelligence official on Tuesday that Syria repeatedly used chemical weapons—the United States has been waiting for the results of an exhaustive analysis by the United Nations of soil, hair and other material to determine whether chemical warfare agents have been used.
But that investigation has been hobbled because the United Nations inspectors have not been allowed into Syria. Also, the scope of that investigation does not extend to who used the weapons, focusing merely on whether chemical agents were used. The United States is also conducting its own assessment, as are Israel and other countries.
Even if the United Nations investigation proves the use of chemicals, an official said, the White House must determine who used them and whether they were used deliberately or accidentally. He did not offer a timetable for that process.
"It is precisely because this is a red line that we have to establish with airtight certainty that this happened," said the official, who spoke on the condition of anonymity so he could discuss internal deliberations. "The bar on the United States is higher than on anyone else, both because of our capabilities and because of our history in Iraq."
Defense Secretary Chuck Hagel, speaking in Cairo during a Middle East tour that has been dominated by worries about Syria, said, "Suspicions are one thing; evidence is another."
Some analysts say they worry that if the United States waits too long, it will embolden Mr. Assad, who has steadily escalated the lethality of the weapons used against the opposition. The government's use of chemical weapons in isolated episodes, these experts said, would be a way to test international reaction before using them on a wider scale.
Last August, Mr. Obama threatened the Syrian government with unspecified American action if there was any evidence that chemical weapons were being used or moved on a large scale. On Tuesday, Israel's top military intelligence analyst, Brig. Gen. Itai Brun, said at a security conference in Tel Aviv that the Syrian government had used chemical weapons, and he criticized the international community for not doing more in response.
"The president's red line appears to have been crossed," said Martin S. Indyk, a former American ambassador to Israel. "The administration has to take some time to decide what to do about it."
"But if they end up leaving the impression that the president is not willing to enforce his red line," said Mr. Indyk, who is now at the Brookings Institution, "that will have consequences in the region, particularly when it comes to Iran's nuclear program, as well as for our ability to deter Assad's use of chemical weapons in Syria."
Administration officials said their assessment of chemical weapons in Syria was not much different from that of Britain and France, which sent letters to the United Nations' secretary general, Ban Ki-moon, last month urging a thorough investigation of the accusations.
Although Britain and France laid out allegations of chemical weapons attacks in three places in Syria, neither country said it was certain that chemical weapons had been used, according to copies of the letters obtained by The New York Times.
Even within Israel, the military's assessment has not been fully embraced by government officials and analysts who follow Syria. Several officials said Wednesday that while they did not doubt the evidence, they worried that the general's speech would be used to pressure Washington.
"Every intelligence branch can submit its own assessment," said an Israeli official, who spoke on the condition of anonymity. "The issue of chemical weapons is being examined by Israel and the United States at the most senior levels, and is still being discussed."
Another official said that was the reason that Prime Minister Benjamin Netanyahu told Mr. Kerry on Tuesday that he could not confirm the assessment.
"There's a difference between what the I.D.F. feels is the truth as they see it and what we feel is appropriate for the dialogue between the two governments," he said, referring to the Israel Defense Forces. "Don't read into this an effort to force America's hand."
Mr. Hagel, in Egypt, declared that Washington would not be rushed into action by foreign intelligence reports, even those from allies. The administration, he said, has to be "very careful" before drawing conclusions and, if necessary, changing its policy, and should await a full review by United States intelligence agencies.
Administration officials said that the Pentagon had prepared a menu of military options for Mr. Obama if he concluded that there was incontrovertible evidence that chemical weapons had been used. Those options, one official said, could include missile strikes on Syrian aircraft from American ships in the Mediterranean or commando raids.
Last fall, the United States military secretly sent a task force of 150 planners and other specialists to Jordan to help deal with an influx of refugees from neighboring Syria, as well as the possibility that Syria could lose control of its chemical weapons stockpiles.
On Tuesday, at a NATO meeting in Brussels, Mr. Kerry said that the alliance should plan for the possibility of a chemical weapons attack by Syria. Turkey, a NATO member that borders the country, would be most at risk from such an attack. Mr. Kerry later clarified that he had not been calling for a specific NATO role in responding to Syria.
Experts on chemical warfare said the administration's methodical approach was warranted. The evidence that has emerged so far is suggestive of chemical attacks, they said, but not conclusive. Syrian government forces could have used riot-control gas that, while extremely powerful, does not qualify as a chemical warfare agent, like sarin.
"It's not a smoking gun, at least so far," said Keith Ward, an expert on chemical warfare who worked for the Department of Homeland Security and the Navy and is now advising Human Rights Watch.
Critics, while acknowledging the murkiness of the situation, said the White House was setting the bar too high. "They're not going to be able to have that smoking gun," said Andrew Tabler, a Syria expert at the Washington Institute for Near Eastern Policy.