Syria chemical weapons use likely says United States Amateur video out of Syria shows the aftermath of an airstrike in Idlib province as the US weighs evidence of chemical weapon use. Smoke rises after shelling on al-Turkman mountains in the Latakia province, western Syria on April 25, 2013. THE White House says Syria had likely used chemical weapons against rebel forces on a "small scale," but emphasized US 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. 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. Ms 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 Defence 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." In London, Britain's Foreign Office too said it 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. His 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," the letter said. 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. Last month, during a historic visit to Israel, Mr Obama said the use of such weapons would be a "grave and tragic" mistake on Assad's behalf and that it would be a "game changer." Asked if the intelligence assessment meant that Syria had passed the declared "red line," Mr Hagel said that was a policy question and that his task was to provide the US president with "options." Chemical weapons are "uncontrollable deadly weapons" that most leaders view as being in a "different category," Hagel said at the end of a Middle East tour with meetings in Israel, Jordan, Saudi Arabia, Egypt and the United Arab Emirates. Senior US Senator John McCain, who was one of the lawmakers who received a letter from the administration about Syria, said the key now was to ensure chemical weapons did not fall into the wrong hands. "Some of them are in heavily contested areas and could easily fall into the hands of jihadist extremists," he told CNN.