The United States has said Syria had likely used chemical weapons against rebel forces. However, US intelligence agencies are hesitant to fully confirm the results of the assessment.
US intelligence agencies believe Syria's government has probably used chemical weapons on a limited scale, the White House announced Thursday.
"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 is based in part on "physiological samples," points to the possible use of sarin, a man-made nerve agent, which can cause convulsions, respiratory failure and death.
However, Hayden also said that the matter of who had been in possession of the weapons at various stages 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. France, Britain and Israel have also pointed to evidence suggesting chemical arms have been used in Syria's conflict.
The US had been investigating reports that Syrian President Bashar al-Assad's forces had used chemical weapons, a move President Barack Obama has said would cross a "red line," leading to possible military action. However, officials said Obama needed "credible and corroborated" facts.
'Violation of every convention'
US Defense Secretary Chuck Hagel, speaking in Abu Dhabi at the end of a Middle East tour, 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."
Asked if the assessment meant that Syria had passed the declared "red line," Hagel said his task was to provide the US president with "options."
The Obama administration explained 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.
The assessment triggered immediate calls for US action by members of Congress who advocate deeper US involvement.
British sarin suspicions
Britain's Foreign Office said on Thursday it had information that showed chemical weapons had been probably used in Syria. London urged al-Assad to work with international bodies to prove that he had not sanctioned their use.
"We have limited but persuasive information from various sources showing chemical weapon use in Syria, including sarin. This is extremely concerning. Use of chemical weapons is a war crime," a Foreign Office spokesman said in a statement.
Syria is not a party to the Chemical Weapons Convention, which bans their use. For months, the United States, Israel and European countries have expressed concern over how well Syrian arms stockpiles, including any potential chemical weapons, were being guarded.
The only other states not to have signed the international treaty are Israel, Myanmar, Angola, Egypt, North Korea, Somalia and South Sudan.
hc/ rc (AFP, Reuters, dpa)