The White House said Thursday that the U.S. believes "with some degree of varying confidence" the Syrian government has used chemical weapons — specifically the nerve agent sarin — against its own people.
A letter from the White House to members of Congress said the assessment was based on "physiological samples" but called for a United Nations probe to corroborate it and nail down when and how they were used.
"We are continuing to do further work to establish a definitive judgement as to whether or not the red line has been crossed and to inform our decision-making about what we'll do next," a White House official said.
"All options are on the table in terms of our response," the official added.
Secretary of State John Kerry told reporters at the Capitol that the U.S. believes chemical weapons were used twice, but the letter doesn't specify that.
"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," the letter said.
"We do believe that any use of chemical weapons in Syria would very likely have originated with the Assad regime," it added.
"Thus far, we believe that the Assad regime maintains custody of these weapons, and has demonstrated a willingness to escalate its horrific use of violence against the Syrian people."
Defense Secretary Chuck Hagel said he had not seen the evidence supporting the assessment, but added that use of chemical agents "violates every convention of war."
Sarin is a man-made nerve agent that has been used in terrorist attacks in Japan and possibly during the Iran-Iraq War in the 1980s, according to the U.S. Centers for Disease Control. In large doses, it can cause convulsions, paralysis and death.
The U.S. has long believed that Syria was stockpiling chemical weapons. Intelligence reports indicate that it has sarin and the nerve agent tabun along with traditional chemicals like mustard gas and hydrogen cyanide. A 2011 CIA report said Syria was also developing the potent nerve agent VX, which could render a city uninhabitable for days.
Syria's information minister, Omran al-Zoubi, said in an interview with Russian TV that the government has not and will not use chemical weapons and blamed potential evidence of their existence on "armed terrorist groups," the state news agency reported.
A spokesman for the rebel Free Syrian Army, Fahd Almasri, claimed Syria has launched chemical attacks in nine places and was poised to do so again at the Lebanon border and in Damascus "when Assad knows he is finished."
"Now is the moment to find a solution very quickly," Almasri told NBC News in a phone interview.
President Obama has said the verified use of chemical weapons by the regime would be a "red line" and a "game-changer" for U.S. and international military intervention in the Syrian civil war.
"Precisely because the President takes this issue so seriously, we have an obligation to fully investigate any and all evidence of chemical weapons use within Syria," said the letter, which was signed by Obama's legislative director, Miguel Rodriguez.
The letter was a response to a request from a bipartisan group of senators who asked the White House for answers after the Israeli military’s top intelligence analyst cited photographs of people "foaming from the mouth” as evidence of chemical weapons use.
Sen. Bob Corker, R-Tenn., ranking member of the Senate Foreign Relations Committee, called the development “deeply troubling.”
“While more work needs to be done to fully verify this assessment…it is becoming increasingly clear that we must step up our efforts,” Corker said.
“I should make clear, however, that it if it comes to the use of military force, before the president takes any action to commit U.S. forces to any effort in Syria or elsewhere, I expect him to fully consult with the Senate and seek an authorization for the use of military force."
Sen. Dianne Feinstein, chair of the Senate Intelligence Committee, said the assessment could spark a dangerous reaction from Damascus.
"I am very concerned that with this public acknowledgement, President Assad may calculate he has nothing more to lose and the likelihood he will further escalate this conflict therefore increases," Feinstein said.
The White House official called for a high level of scrutiny — but also caution.
"Given our own history with intelligence assessments, including intelligence assessments related to weapons of mass destruction, it's very important that we are able to establish this with certainty and that we are able to present information in a way that is airtight," the official said.
NBC News' Kasie Hunt, Kelly O'Donnell, Robert Windrem and Charlene Gubash contributed to this story