The assessment, based on visual evidence of alleged attacks, could raise pressure on the U.S. 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 U.S. 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."
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.
He cited images of people hurt in the alleged attacks, but gave no indication that he had other evidence, such as soil samples, typically used to verify chemical weapons use.
"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," Brun 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 defense missiles.
Brun appeared to lament the lack of response by the international community, saying the silence could encourage rogue groups that do not play by traditional rules of war.
"The fact that chemical weapons were used without an appropriate response is a very disturbing development because it could signal that such a thing is legitimate," he said. "I think we need to be very worried that chemical weapons will reach elements that are less responsible."
He delivered his assessment as U.S. Defense Secretary Chuck Hagel was wrapping up a visit to Israel.
Reacting to Brun's comments, Pentagon spokesman George Little said the U.S. "continues to assess reports of chemical weapons use in Syria."
"The use of such weapons would be entirely unacceptable," he added." ''We reiterate in the strongest possible terms the obligations of the Syrian regime to safeguard its chemical weapons stockpiles, and not to use or transfer such weapons to terrorist groups like Hezbollah."
Israel, which borders southwestern 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 militant 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. Hezbollah battled Israel to a monthlong stalemate in 2006.
Israeli Prime Minister Benjamin Netanyahu on Monday repeated that Israel has the right "to defend itself" against any threat. He spoke at a meeting Hagel.
At a meeting with Hagel on Monday, Israel's defense 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.
The Israeli military has fired at targets inside Syria on several occasions in response to gunfire or mortar shells landing in the Israeli-occupied Golan Heights. Israel has all but admitted that it carried out an airstrike in Syria in January that destroyed a shipment of anti-aircraft missiles believed to be headed to Hezbollah.
"We proved it. When they crossed these red lines, we operated, we acted," Yaalon said.
While Israel has focused on the dangers of militants obtaining chemical weapons, the West has expressed a broader concern about any use of chemical weapons in Syria.
Britain and France informed U.N. Secretary-General Ban Ki-moon last month that they have reliable evidence Assad's forces used chemical weapons that caused injuries and deaths. They cited soil samples and interviews with witnesses and opposition figures.
The two countries asked the U.N. chief to investigate allegations of chemical weapons use in two locations near Damascus on March 19, as well as in the city of Homs on Dec. 23. Ban has appointed a team of chemical weapons experts to investigate the allegations, but the Syrian government has largely blocked the team from doing its work. Syria, meanwhile, has accused rebels of using chemical weapons.
The chairman of the Joint Chiefs of Staff, Gen. Martin Dempsey, acknowledged last week that he could not guarantee that U.S. forces could secure the chemical weapons caches within Syria. He said Syria has been moving the weapons between numerous sites.
Syria is believed to have hundreds, perhaps thousands, of tons of chemical agents, including mustard gas, a blistering agent, and the more lethal nerve agents, sarin and VX.
During a trip to Israel last month, Obama warned that the use of chemical weapons would be a "game changer." He said if confirmed, "the international community has to act on that additional information," but did not detail what the next steps would be.
Amos Yadlin, a retired chief of Israeli military intelligence who heads the Institute for National Security Studies that hosted Tuesday's conference, urged the U.S. to intervene. He said the red line in Syria had been crossed and that the U.S. should act.
Not doing so would "strengthen the opinion of those, particularly in Israel, who are suspicious of America's commitment when red lines are crossed," he said. He suggested the U.S. impose a no-fly zone over Syria or even initiate a bombing campaign similar to the one conducted against the regime of late Libyan dictator Moammar Gadhafi.
"So far they haven't applied any leverage to Bashar, and this has allowed him to do what Gadhafi could never do," Yadlin told The Associated Press.