Syria's opposition called on the international community on Thursday to act "urgently and decisively" on President Barack Obama's public warning that Syria's use of chemical weapons would cross a "red line."
The call came after the White House and other top officials said that U.S. intelligence has concluded with "varying degrees of confidence" that the Syrian government has used chemical weapons in its civil war.
A statement by the Syrian National Coalition called on the world community to prove to the Syrian regime that the statements about crossing a red line were not just "empty words."
The statement added that failure to act will be seen by the regime as a green light to use chemical weapons on a larger scale in the future.
The United States said Thursday for the first time that Syria had likely used chemical weapons against rebel forces, but emphasized spy agencies were still not 100 percent sure of the assessment.
A senior White House official subsequently said that "all options are on the table" should use of the chemical weapons be confirmed.
U.S. Defense Secretary Chuck Hagel, speaking in Abu Dhabi, said the decision to release the intelligence report had been "made within the past 24 hours" and warned that use of such weapons "violates every convention of warfare."
A U.S. defense official traveling with Hagel confirmed that the phrase "varying degrees of confidence" is a term commonly used by the intelligence community to indicate disagreement among various agencies.
Earlier this week, an Israeli general in military intelligence alleged that Syria had used chemical agents more than once during the protracted civil war, after Britain and France had voiced similar concerns to the United Nations.
U.S. senators said on Thursday it was time to intervene in Syria in the wake of the latest intelligence report.
Sen. Robert Menendez, Chairman of the Senate Foreign Relations Committee, said in a statement that Assad had “crossed a red line” and that the international community should act to ensure his fall.
Senior U.S. Senator John McCain insisted that the U.S. military should have already intervened in Syria.
"We should have intervened long ago, whether (Bashar al-Assad) was using them (chemical weapons) or not. He slaughtered 80,000 people while we sat by and watched. It's been one of the most shameful chapters in American history," he said.