A senior Israeli military intelligence official has claimed Syrian President Bashar Assad used chemical weapons last month in his battle against insurgent groups.
It was the first time Israel has accused the embattled Syrian leader of using his stockpile of non-conventional weapons.
The assessment could raise pressure on the US and other Western countries to intervene in the Syrian conflict. Britain and France recently announced that they had evidence that Assad's government had used chemical weapons. Although the US says it has not been able to verify these claims, President Barack Obama has warned that the introduction of chemical weapons by Assad would be a "game changer."
Israel, which borders south-western Syria, has been warily watching the Syrian civil war since the fighting erupted there in March 2011. Although Assad is a bitter enemy, Israel has been careful not to take sides, partly because the Assad family has kept the border with Israel quiet for the past 40 years and partly because of fears of what would happen if he is toppled.
Israeli officials are especially concerned that Assad's stockpile of chemical weapons and other advanced arms could reach the hands of Assad's ally, the Hezbollah guerrilla group in Lebanon, or Islamic extremist groups trying to oust him. The concern is that if Assad is overthrown, any of these groups could turn his sophisticated arsenal against Israel.
In his assessment, Brig Gen Itai Brun, the head of research and analysis in Israeli military intelligence, told a security conference in Tel Aviv that Assad has used chemical weapons multiple times. Among the incidents were attacks documented by the French and British near Damascus last month.
"To the best of our professional understanding, the regime used lethal chemical weapons against the militants in a series of incidents over the past months, including the relatively famous incident of March 19," he said. "Shrunken pupils, foaming at the mouth and other signs indicate, in our view, that lethal chemical weapons were used."
He said sarin, a lethal nerve agent, was probably used. He also said the Syrian regime was using less lethal chemical weapons, and that Russia has continued to arm the Syrian military with weapons such as advanced SA-17 air defence missiles.
Israeli Prime Minister Benjamin Netanyahu repeated that Israel has the right "to defend itself" against any threat. He spoke at a meeting with visiting US Defence Secretary Chuck Hagel. on Monday
At a meeting with Mr Hagel, Israel's defence minister, Moshe Yaalon, laid out a number of "clear red lines" to Syria that could trigger an Israeli response. Among them were transferring sophisticated weapons to Hezbollah and other "rogue elements" in Syria, cross-border attacks into Israel or "rogue elements" getting hold of Syrian chemical weapons.