The White House said Thursday that military forces loyal to Syrian President Bashar Assad probably used chemical weapons on a "small scale," reigniting the debate over what role the U.S. should play in trying to topple the regime.
In a carefully worded letter to senators, the White House said the details are still sketchy, and an administration official briefing reporters said it was not clear whether Syria had crossed the "red line" President Obama drew, threatening consequences for using chemical weapons.
"Our intelligence community does assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin," Miguel E. Rodriguez, director of the White House Office of Legislative Affairs, said in the letter.
The revelation confirms claims made recently by authorities in Israel, Britain and France.
Mr. Rodriguez said the assessment that chemical weapons may have been used was "based in part on physiological samples."
But the administration is wary of potentially faulty findings, and the senior administration official who briefed reporters — speaking on the condition of anonymity — cautiously referenced faulty intelligence assessments on Iraqi weapons of mass destruction that the Bush administration used to justify the 2003 invasion of Iraq.
Still, the development set the stage for Washington to significantly deepen its involvement in the civil war that has gripped Syria for two years, where more than 70,000 people are believed to have been killed since March 2011.
The White House has avoided getting drawn into the conflict. Mr. Obama has resisted calls by his own Cabinet members and some lawmakers to directly arm Syrian opposition groups. The president's reasoning, which also has some supporters in Washington, has centered on the notion that the risk of U.S. weapons ending up in the hands of Islamist elements in the Syrian opposition is simply too high.
Some lawmakers, particularly Republicans, have argued that the administration's policy has enabled Mr. Assad to remain in power far longer than necessary. House Speaker John A. Boehner, Ohio Republican, said Thursday that Mr. Obama now must take a firmer stance.
"It's past time for the president to have a robust conversation with the Congress and the American people about how best to bring Assad's tyranny to an end," Mr. Boehner said.
The speaker's remarks were echoed by Dianne Feinstein, California Democrat and chairwoman of the Senate Select Committee on Intelligence, who said she thinks the "red line" that Mr. Obama drew on chemical weapons use has been crossed. She said the U.S. and the international community must respond forcefully.
"Action must be taken to prevent larger-scale use" of such weapons, Mrs. Feinstein said in a statement. "The world must come together to prevent this by unified action that results in the secure containment of Syria's significant stockpile of chemical weapons."
Sen. John McCain, an Arizona Republican who has long advocated deeper U.S. involvement in the conflict and called for the imposition of a no-fly zone over Syria, agreed, although he voiced doubts that the Obama administration is prepared to act quickly.
"The president clearly stated that it was a red line and that it couldn't be crossed without the United States taking vigorous action," Mr. McCain told Fox News on Thursday.
Others urged a cautious approach.
"I believe Syria's President Assad must go. But I don't feel it's in our best interest to go into Syria right now," said Sen. Bill Nelson, Florida Democrat, who said it's not clear how extensively chemical weapons were used and that it makes more sense for the U.S. to put its efforts into bolstering Syrian rebels.
The White House letter to senators repeated the administration's claim that "the use of chemical weapons — or transfer of chemical weapons to terrorist groups — is a red line for the United States."
But the letter did not spell out specific consequences, instead saying that the U.S. and the international community "have a number of potential responses available, and no option is off the table."
Secretary of State John F. Kerry also has resisted any specific characterization of what such a response might involve. Speaking to representatives of NATO member nations this week, Mr. Kerry said the international community should "carefully and collectively consider how NATO is prepared to respond to protect its members from a Syrian threat, including any potential chemical weapons threat."
The White House letter, meanwhile, underscored the administration's belief that any use of chemical weapons in Syria very likely would have originated with the Assad regime, rather than by opposition rebels.
But, the letter stressed, U.S. authorities are still attempting to clarify precisely how and where chemical agents may have been deployed and used during recent months in Syria.
"The chain of custody is not clear, so we cannot confirm how the exposure occurred and under what conditions," Mr. Rodriguez wrote.
The result now finds the United Nations poised to test soil samples from Syria to help clarify whether and how chemical weapons may have been used. London's Guardian newspaper reported Thursday that the samples were being provided to the U.N. by Western intelligence agencies.
U.N. officials have sought for weeks to conduct such tests inside Syria, but the Assad government has blocked their access to places where chemical weapons reportedly have been used.
The senior administration official who spoke on the condition of anonymity Thursday said the White House is simply not comfortable taking action based solely on the intelligence assessments outlined Thursday, and that the U.S. is still looking for "firm evidence."
When asked specifically whether Mr. Obama's "red line" had been crossed, the official said the U.S. was working to corroborate that information "to have firm evidence" when consulting with allies and countries in the region to determine a response.
But the official said Mr. Assad should now know that the U.S. is monitoring the situation closely.
⦁ Susan Crabtree, Kristina Wong and Sean Lengell contributed to this report.
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