JERUSALEM - For the first time Israel has accused the forces of President Bashar al-Assad in Syria of using chemical weapons against rebel forces over the past several months, including "probably sarin," the deadly nerve agent that is one of the most feared elements in Syria's formidable chemical weapons arsenal.

The accusation was made today by Brigadier General Itai Baron, the head of the Research Division of Israel's Military Intelligence, and it ratchets up the international allegations against Syria of chemical weapons use, none of which have previously mentioned sarin.

"To the best of our professional understanding, the regime has caused death using chemical weapons against [rebel forces] over the last few months," said Baron at an international security conference.

He spoke specifically about an incident on March 19 near the northern city of Aleppo in which 26 people - many of them regime soldiers - were killed in what appeared to be a chemical attack. The Assad regime immediately blamed rebels, who in turn accused the regime.

"What chemical weapons specifically" were used? Baron asked. "Probably sarin."

Baron cited as evidence photos he'd seen of the area and victims with "constricted pupils, the foam coming out of mouths, additional signs."

Britain and France reportedly told United Nations Secretary General Ban Ki-Moon last week that the Syrian regime has used chemical weapons, including on March 19. They based their findings, they said, on soil samples and interviews with witnesses and doctors. The U.N. had been invited into Syria by the regime to investigate the March 19 incident but when it asked to do a broader investigation into other allegations, Syria blocked their entry.

The U.S. has been less aggressive in its accusations of chemical weapons use, likely not wanting to paint itself into a corner after repeatedly saying that chemical use would be a "game changer" and "cross a red line," triggering an as-yet undefined action. When asked about the March 19 incident during his visit to Israel in March, Obama responded that the U.S. was still investigating and that he was skeptical the rebels had staged the chemical attack.

Today's Israeli allegation came on the final day of a trip to Israel by Secretary of Defense Chuck Hagel, a trip highlighted by the signing of a $10 billion arms package for Israel. Soon after he left, the Pentagon responded to Baron's remarks in a written statement saying that "the United States continues to assess reports of chemical weapons use in Syria. The use of such weapons would be entirely unacceptable."

Videos have surfaced online of Islamist rebel fighters with vast supplies of chemicals, carrying out experiments on animals and saying they will use chemical weapons against the Assad regime.

The Assad regime is believed to have one of the biggest stockpiles of chemical weapons in the world which contains the VX nerve agent and mustard gas, in addition to sarin.