US Secretary of Defence Chuck Hagel on Thursday said the US intelligence community has "some degree of varying confidence" that the Syrian regime has used chemical weapons, dpa reported.
Syria's use of chemical weapons was on a "small scale" and involved "specifically the chemical agent sarin," Hagel told reporters in Abu Dhabi.
"The decision to reach this conclusion was made in the past 24 hours," he said.
Hagel said the US could not confirm the origin of the weapons. "We do believe that any use of chemical weapons in Syria would very likely have originated with the Assad regime."
Israel, France and Britain have indicated they believe Syrian leader Bashar al-Assad has used chemical weapons, a charge also made by the opposition rebel forces.
Downing Street on Thursday said it had "limited but persuasive information" that chemical weapons - including sarin - have been used in Syria.
"Use of chemical weapons is a war crime. We have briefed our allies, partners and the UN on this information and we are working actively to get more and better information," a government spokesman said.
On Wednesday, the head of Israel's military intelligence research division, Itai Brun said, "In our estimate, the regime has used, and is using, chemical weapons."
Pictures of victims with contracted pupils and foaming mouths indicate that deadly nerve gas - "probably sarin" - had been used by Syrian government forces, Brun said.
Russia has cautioned on the latest chemical weapons claims in Syria, saying previous claims had turned out to be false.
The US has refused to arm the rebels in their two-year bid to oust the al-Assad. But President Barack Obama has declared that any evidence that the regime has used chemical weapons would be a "game changer."
Republican Senators John McCain and Lindsey Graham each declared that the "red line has been crossed" and called for establishment of a no-fly zone over Syria.
"I think it's time ... to provide a safe area for the opposition to operate," McCain said on television. "I think it's pretty obvious that the red line has been crossed."
The White House urged caution and further corroboration of the findings on chemical weapons.
"The chain of custody is not clear, so we cannot confirm how the exposure occurred and under what conditions," the White House said in a letter to McCain and another senator informing them of the situation.
"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," Miguel Rodriguez, an assistant to Obama for legislative affairs, wrote in the letter.
The White House said experience has proved that intelligence assessments alone are not sufficient for weighty decisions - a possible reference to the trumped up intelligence evidence presented by the administration of president George W Bush to justify invading Iraq in 2003.
"Given the stakes involved ... only credible and corroborated facts that provide us with some degree of certainty will guide our decision making and strengthen our leadership of the international community," the White House wrote.