Syrian government forces have used chemical weapons - probably nerve gas - in their fight against rebels trying to force out President Bashar al-Assad, the Israeli military's top intelligence analyst said on Tuesday.
The assessment met with scepticism from the United States, which has declared any use of chemical weapons in Syria's two-year-old civil war a "red line" that could trigger intervention.
U.S. Secretary of State John Kerry said he spoke to Israeli Prime Minister Benjamin Netanyahu and the latter "was not in a position to confirm" the briefing given by Itai Brun, a military intelligence brigadier-general, at a Tel Aviv conference.
"I don't know what the facts are," Kerry told reporters in Brussels.
Netanyahu's office declined comment on Kerry and Brun's remarks, made a day after U.S. Defense Secretary Chuck Hagel said while visiting Israel that Washington's spy agencies were still assessing whether such weapons had been employed.
"To the best of our understanding, there was use of lethal chemical weapons. Which chemical weapons? Probably sarin," Brun told Tel Aviv University's Institute for National Security Studies in the most definitive Israeli statement on the issue.
Forces loyal to Assad were behind the attacks on "armed (rebels) on a number of occasions in the past few months, including the most reported incident on March 19", Brun said.
The Syrian government and rebels last month accused each other of launching a chemical attack near the northern city of Aleppo.
Brun's comments were likely to deepen international concerns over events in Syria. Kerry said separately on Tuesday 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".
Speaking with a Powerpoint presentation showing what appeared to be a wounded or dead child, Brun said that foam coming out of victims' mouths and contracted pupils and "other signs" indicated deadly gas had been used.
Another Israeli military officer with knowledge of Brun's briefing said it drew on secret intelligence other than material available in the public domain.
"When an authority as senior as Brun makes such a statement in public, you can be sure it is based on solid evidence," the officer told Reuters on condition of anonymity.
Ralf Trapp, an independent consultant on chemical and biological weapons arms control based in Geneva, said the symptoms described by Israeli intelligence were "consistent with sarin gas," but photographic evidence alone was not conclusive.
Asked about Brun's remarks, Pentagon spokesman George Little signalled no change in the official U.S. line: "The United States continues to assess reports of chemical weapons use in Syria. The use of such weapons would be entirely unacceptable."
On Monday, Hagel said the use of chemical weapons by Assad's forces would be a "game changer" and the United States and Israel "have options for all contingencies".
Hagel met Netanyahu on Tuesday, a day after flying in an Israeli military helicopter over the occupied Golan Heights on the edge of the fighting in Syria.
"This is a difficult and dangerous time, this is a time when friends and allies must remain close, closer than ever," Hagel, in remarks to reporters before his talks with Netanyahu, said of the United States and Israel.
He travelled to Jordan, home to hundreds of thousands of Syrian refugees, for talks with Prince Faisal, King Abdullah's brother, and General Mashal al-Zaben, the country's military chief, and then on to Saudi Arabia for meetings with Crown Prince Salman bin Abdulaziz al Saud, the defense minister.
Discussions between Syria and the United Nations on a U.N. investigation of possible use of chemical weapons have been at an impasse due to the Syrian government's refusal to let the inspectors visit anywhere but Aleppo, diplomats and U.N. officials said last week.
U.N. diplomats said Britain and France had provided U.N. Secretary-General Ban Ki-moon's office with what they believed to be strong evidence that chemical weapons also had been used in the city of Homs.
Israel, which has advanced intelligence capabilities that it shares with its Western allies, has voiced concern that parts of Syria's chemical arsenal would end up in the hands of jihadi fighters or the Lebanese guerrilla group Hezbollah, with which it waged a war in 2006.
Israeli leaders have cautioned they will not allow that to happen. In an attack it has not formally confirmed, Israeli planes bombed an arms convoy in Syria in February, destroying anti-aircraft weapons destined for Hezbollah.
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