The forces of Syrian President Bashar Assad have used chemical weapons against combatants, an Israeli Military Intelligence official said Tuesday.

"To the best of our professional understanding, the regime has used lethal chemical weapons on a number of occasions, including the incident on March 19," the head of the Research Division at Military Intelligence, Brig. Gen. Itai Brun, said at conference of the Institute for National Security Studies.

Prime Minister Benjamin Netanyahu did not confirm the assessment. U.S. Secretary of State John Kerry, who held a phone conversation with the Israeli leader, also refrained from any confirmation.

In his remarks to the INSS, Brun addressed the characteristics that led IDF Military Intelligence to this conclusion, photographs taken of the affected areas after the attacks, including depictions of foam coming out of the mouths of Syrians in the area of the attacks.

Brun added that the Israeli intelligence community believed that the chemical weapon used was sarin-based.

At the end of his lecture, Brun was asked by the head of INSS, Maj. Gen. (res.) Amos Yadlin, who served in the past as director of Military Intelligence, about his assessment of the likelihood of the Assad regime using chemical weapons against Israel and whether Syrian rebels would use chemical weapons should they take over chemical weapon stocks.

"In Syria there is a large arsenal of chemical weapons, more than 1,000 tons of chemicals, thousands of aerial bombs and quite a lot of warheads and surface-to-surface missiles that can be armed with chemical weapons," said Brun.

Brun also criticized the global community's response to events in Syria. "The response of the world on this issue reflects the same trend of limited influence and a predisposition not to intervene," said Brun. "The developments are certainly worrying ones: First the fact that chemical weapons have been used without any… [international] response is a very worrying development and could certainly signal that such a thing is legitimate."

He also stated that there was a possibility that Syria's chemical weapons would find their way into the hands of terrorists, "who don’t undertake normal cost-benefit calculations," something he also called, "worrying." All the same, Brun qualified his words by saying that he wasn't certain that regional upheaval would lead to the use of chemical weapons, but that there would be "a need to see how the situation develops in the short term."

Brun also addressed attempts to transfer advanced weapons systems from Syria to Hezbollah. Regarding what foreign media recently described as an Israeli attack on Syria, Brun said, "the SA-17 missile launchers that were bombed in Syria were going to be transferred to Hezbollah." Defense Minister Moshe Ya'alon also addressed the issue on Monday when he said that the transfer of advanced weaponry was crossing a red line and when such a thing was done, "we act."

The international response

White House spokesman Jay Carney told reporters on Tuesday that the United States has not concluded that the Syrian government used chemical weapons against its own people as a way to try to crush the rebellion in the country, .

.The United States has declared that use of chemical weapons by Syria would cross a "red line" that could trigger intervention.

Pressed on the issue at his daily news briefing, Carney said the United States had not concluded that chemical weapons had been used and said it is difficult to determine when such weapons are used.

He said the United States remains concerned about reports that these weapons had been utilized and that Washington is skeptical of reports that the Syrian opposition had used them.

Obama has made clear that the use of chemical weapons or transmission of them to "non-state actors" is unacceptable.

U.S. Secretary of State John Kerry said earlier Tuesday that Israeli Prime Minister Benjamin Netanyahu, with whom he had spoken with by phone, could not confirm Brun's comments.

"I don't know yet what the facts are, I don't think anyone knows what they are," Kerry told reporters in Brussels after an Israeli Military Intelligence official revealed the information at a conference of the Institute for National Security Studies.

"I think it's fair for me to say that [Prime Minister Benjamin Netanyahu] was not in a position to confirm that in the conversation that I had," Kerry said..

"The information I have at this point does not confirm it to me in a way that I would be comfortable commenting on it as fact," Kerry said, adding: "Obviously whatever allegations were made have to be thoroughly investigated."

Kerry also said that NATO needed to consider how practically prepared it was to "respond to protect its members from a Syrian threat, including any potential chemical weapons threat."

Washington has reiterated that the use of chemical weapons by the Assad regime would constitute an unequivocal red line, which if crossed is liable to spur American involvement in the crisis.

“We will not tolerate the use of chemical weapons against the Syrian people,” U.S. President Barack Obama said in his address last month at the Jerusalem International Convention Center, two days after the use of chemical weapons was first reported. “The world is watching, and we will hold you accountable.

Russian Foreign Minister Sergei Lavrov wants experts to assess claims that chemical weapons have been used in Syria, noting that prior allegations were proven wrong.

"We believe that any information that these weapons were used should be studied carefully, in the same way as it happened a couple of weeks ago when the information appeared that chemical weapons were used close to Aleppo," Lavrov told journalists at NATO meeting in Brussels.

"However, a bit later our American colleagues said that it was not chemical weapons, but chemical hazardous substances that were used," he added. "Any information that chemical weapons were used should be investigated by experts ... on the ground."

Lavrov also accused a "very aggressive and blood-thirsty" minority within the international community of blocking "all efforts to build dialogue" in Syria. He did not identify the target of his criticism.

Recently, Western intelligence officials have said that Syrian forces used chemical weapons in two incidents in the Damascus area on March 19.

However, various intelligence agencies, first and foremost the Americans, have not concluded whether the material used was a toxic chemical or a material that paralyzes only and does not kill.