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“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.