The United States said 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.
US intelligence services had been investigating reports that Syrian President Bashar al-Assad's forces had used chemical arms -- a move Washington has said would cross a "red line," triggering possible military action.
A senior White House official said "all options are on the table" should use of the weapons be confirmed, but a US defense official stressed that a military intervention was not imminent and signaled spy agencies had differing opinions.
"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.
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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.
Meanwhile a watchdog reported fresh fighting on the edges of Damascus on Friday, as troops took on rebels in the north, south and east, backed in some areas by tanks and militia.
"Fierce clashes are raging in Barzeh district pitting rebels against troops and members of the pro-regime popular committees," the Syrian Observatory for Human Rights said.
The north Damascus neighbourhood also came under tank fire by the army, the Britain-based group said.
The militiamen were brought in from the Esh al-Warwar district, which has a population mainly drawn from the same Alawite minority as President Bashar al-Assad, it added.
Most of the rebels are from Syria's Sunni Muslim majority.
Analysts say the militia have better knowledge of the ground than the largely conscript army and are more adept at street fighting.
Rebels and troops also fought in several areas of south Damascus. In the east of the capital, fighting broke out in Jobar district, where the rebels hold a number of enclaves, the Observatory said.
The army also shelled the eastern suburbs, where the rebels have rear-bases they have used to launch attacks inside the capital.
In violence on Thursday, at least 130 people died, the Observatory said -- 53 civilians, 27 soldiers and 50 rebels.