Brigadier General Itai Brun, the head of military intelligence research at the Israeli Defence Force, told a security conference in Tel Aviv that "there is a huge arsenal of chemical weapons in Syria".
"Our assessment is that the [Assad] regime has used and is using chemical weapons," said Brun.
Photographs of victims, which showed them foaming at the mouth and with contracted pupils, were signs that gas had been used, Brun said at a conference organised by the Institute of National Security Studies. "To the best of our understanding, there was use of lethal chemical weapons. Which chemical weapons? Probably sarin."
He specifically referred to March 19 among "a number of incidents" in which chemical weapons had been used by the regime and criticised the lack of response from the international community.
"The regime has increasingly used chemical weapons" from its "huge arsenal," Brun said. "The very fact that it has used chemical weapons without any appropriate reaction, this is a very worrying development, because it might signal that this is legitimate."
The British and French governments said in letters to United Nations secretary general Ban Ki-moon last week that there was credible evidence that Syria had used chemical weapons since December in or near the cities of Homs, Aleppo and Damascus.
President Bashar al-Assad's regime claimed that opposition forces used chemical weapons last month. State television stated that more than 30 people had been killed in an attack near Aleppo after "terrorists fired rockets containing chemical materials".
Opposition activists said regime forces had fired poison gas, hitting their own troops and nearby civilians.
On a visit to Israel last month, United States President Barack Obama said the deployment of chemical weapons inside Syria could be a "game changer" for the US.
Last week, Israeli Prime Minister Binyamin Netanyahu said Israel would defend itself if Syria's chemical and anti-aircraft weapons fell into the hands of Hezbollah or jihadists.
"We are prepared to defend ourselves if the need arises and I think people know that what I say is both measured and serious," he told the BBC.
Brun echoed Netanyahu's comments at Tuesday's conference, saying: "We have to be bothered by the possibility that chemical weapons are going to get into the hands of less responsible actors … It is certainly possible that there will be other incidents of attack against Israel by other organisations that obtain different types of weapons." – © Guardian News and Media Limited 2013