US Secretary of State John Kerry has said the Syrian regime has launched two chemical weapons attacks.
US intelligence had earlier concluded "with some degree of varying confidence" that the Syrian government has twice used chemical weapons in its fierce civil war, the White House and other top administration officials said today.
However, officials also said more definitive proof was needed and the US was not ready to escalate its involvement in Syria.
That response appeared to be an effort to bide time, given President Barack Obama's repeated public assertions that Syria's use of chemical weapons, or the transfer of its stockpiles to a terrorist group, would cross a "red line".
The White House disclosed the new intelligence today in letters to two senators, and Defence Secretary Chuck Hagel, travelling in Abu Dhabi, also discussed it with reporters.
"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 White House said in its letter, which was signed by Mr Obama's legislative director, Miguel Rodriguez.
Shortly after the letter was made public, Secretary of State John Kerry said on Capitol Hill that there were two instances of chemical weapons use.
Mr Hagel said the use of chemical weapons "violates every convention of warfare."
It was not immediately clear what quantity of weapons might have been used, or when or what casualties might have resulted.
Mr Obama has said the use of chemical weapons would be a "game-changer" in the US position on intervening in the Syrian civil war, and the letter to Congress reiterated that the use or transfer of such weapons in Syria was a "red line for the United States." However, the letter also suggested a broad US response was not imminent.
Mr Rodriguez wrote that "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."
The letter went to Senators John McCain and Carl Levin.
The assessment, Mr Rodriguez said, was based in part on "physiological samples."
The letter also said the US believes the use of chemical weapons "originated with the Assad regime." That is consistent with the Obama administration's assertion that the Syrian rebels do not have access to the country's stockpiles.
A senior defence official cautioned that the White House letter was not an "automatic trigger" for policy decisions on the use of military force. The official alluded to past instances of policy decisions that were based on what turned out to be flawed intelligence, such as the Bush administration's decision to invade Iraq after concluding that Saddam Hussein had weapons of mass destruction, including nuclear weapons.
The official commented only on condition of anonymity.
Mr McCain quoted from the letter in making his own comments to reporters on Capitol Hill.
"We just received a letter from the president in response to our question about whether Assad had used chemical weapons," Mr McCain said following a closed briefing with Mr Kerry on Syria and North Korea.