"Our intelligence community does assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria," US National Security Council spokeswoman Caitlin Hayden said.
The assessment, which she said was based in part on "physiological samples," points to the possible use of sarin, a man-made nerve agent used in two attacks in Japan in the 1990s. It can cause convulsions, respiratory failure and death.
Hayden, however, warned the chain of custody of the weapons was "not clear, so we cannot confirm how the exposure occurred and under what conditions."
"Given the stakes involved, and what we have learned from our own recent experience, intelligence assessments alone are not sufficient," she said.
US 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 US 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.
But the assessment reflected a degree of certainty that Syria most likely has fired chemical agents, the official said.
In London, Britain's Foreign Office said it too had "limited but persuasive" evidence of the use of chemical agents in Syria's grinding civil war, which the UN says has left more than 70,000 people dead since it began in March 2011.
Mounting evidence of chemical weapons attacks on fighters battling Assad's regime could increase the pressure on US President Barack Obama -- who has sought to avoid any US military role in the conflict -- to intervene.
"All options are on the table, in terms of our response," a senior White House official said, adding that Washington was consulting with its allies.
The Obama administration laid out the intelligence assessment in a letter to US lawmakers from Miguel Rodriguez, director of the White House office of legislative affairs.
"We do believe that any use of chemical weapons in Syria would very likely have originated with the Assad regime," the letter said.
So far, US intelligence indicates 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," it added.
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.
Syria asked for a UN investigation but has since refused to let a UN team waiting in the region into the country. Assad's government only wants its claims that opposition rebels used chemical arms to be investigated. UN chief Ban Ki-moon has said the team should also look into opposition claims.
"The secretary-general has consistently urged the Syrian authorities to provide full and unfettered access to the team. He renews this urgent call today," said UN spokesman Martin Nesirky.
Last month, during a historic visit to Israel, Obama said the use of chemical weapons would be a "game changer."
The Pentagon has already sent more than 200 troops to Jordan to prepare for a possible joint operation with allies to secure chemical weapons.
Senior US Senator John McCain gave a hawkish response to the intelligence assessment, insisting the US military should have intervened in Syria "long ago, whether (Assad) was using them (chemical weapons) or not."
"I think it's pretty obvious that a red line has been crossed," McCain said.
"He slaughtered 80,000 people while we sat by and watched. It's been one of the most shameful chapters in American history."
Senator Dianne Feinstein, who chairs the Senate Intelligence Committee, said the assessment demanded "unified action" from the international community that leads to the "containment" of Syria's chemical weapons arsenal.
The US statement sent oil prices surging on fears of an escalation of the conflict in Syria. WTI crude for June delivery, New York's main contract, rocketed $3.00 to $94 a barrel after the US statement, before easing slightly to finish at $93.64.