This citizen journalism image provided by Aleppo Media Center AMC which has been authenticated based on its contents and other AP reporting, shows members
This citizen journalism image provided by Aleppo Media Center AMC which has been authenticated based on its contents and other AP reporting, shows members of the free Syrian Army hiding behind scrap metal during an attack against Syrian government forces, in the neighborhood of al-Amerieh in Aleppo, Syria, Sunday, April. 21, 2013. The Syrian opposition called on Hezbollah to withdraw its fighters from the country immediately, as activists said regime troops supported by pro-government gunmen linked to the Lebanese Shiite militant group battled rebels Sunday for control of a string of villages near the Lebanon-Syria border. (AP Photo/Aleppo Media Center AMC) (HOEP)

TEL AVIV, Israel — U.S. officials said Tuesday they haven't seen conclusive evidence supporting an assertion by Israel's top military intelligence analyst that Syrian government forces have used chemical weapons against rebels.

Secretary of State John Kerry and spokesmen from the White House and the Pentagon said investigations haven't produced hard proof that Bashar Assad's regime has used toxic chemicals — a step that President Barack Obama has said would cross a "red line."

"We have not come to the conclusion that there has been that use," White House spokesman Jay Carney told reporters in Washington. "It is precisely because of the seriousness of the use of chemical weapons — and the seriousness with which the president made clear that that use would be unacceptable — that it is incumbent upon us and our partners to investigate thoroughly and validate or verify allegations of chemical weapons use."

Israeli Brig. Gen. Itai Brun, chief of research and analysis for the army's military intelligence division, cited photographic and other forms of evidence Tuesday indicating Syria used chemical weapons last month near Damascus and Aleppo.

"To the best of our professional understanding, the regime used chemical weapons against fighters in a series of incidents in recent months," Brun said at a conference at Tel Aviv University's Institute for National Security Studies, according to a transcript of his remarks provided by the army.

"The dilated pupils, the frothing at the mouth and other signs testify, in our view, to the use of liquid chemical weapons, some kind of liquid chemical weapons, apparently sarin," Brun said.


Kerry told reporters that Israeli Prime Minister Benjamin Netanyahu had been unable to confirm the reports in a phone call Tuesday.

"I talked to Prime Minister Netanyahu this morning from here," Kerry said at a gathering of the North Atlantic Treaty Organization in Brussels. "I think it's fair to say he was not in a position to confirm that, so it's up to him and their process as to when and how they do that."

"I don't know yet what the facts are," Kerry said. "I don't think anybody knows what they are yet."

Pentagon spokesman George Little said U.S. officials "believe Syria's chemical weapons stockpile remains under Syrian government control." The United States will work with allies to "monitor, investigate and verify any credible allegations," Little said.

American intelligence agencies studying Syria agree that there is little or no credible, verified evidence that the Assad regime has used chemical weapons other than crowd-control agents that aren't classified as weapons of mass destruction, said two U.S. officials who spoke on the condition of anonymity to discuss classified matters.

The officials said some analysts consider it unlikely that Assad would cross Obama's red line except as a last, desperate measure, perhaps to clear a path to exile for his Alawite elite or to a last redoubt in the sect's homeland in northwestern Syria.

The differing U.S. and Israeli assessments, one of the officials said, may reflect varying estimates of how close the regime is to collapse, the likelihood that Islamic extremists could fill the resulting vacuum and the prospects that Sunni fighters tied to al-Qaida or Iranian ally Hezbollah could inherit some or all of Assad's chemical and biological weapons.

Britain and France also have alleged that Assad's troops have used chemical weapons against rebel fighters.

More than 70,000 people have been killed in the uprising against Assad that has lasted more than two years, according to a United Nations estimate.

Obama said when he visited Israel last month that proof Syria used chemical weapons would change U.S. calculations in dealing with the conflict, without specifying what actions he would take. U.S. Defense Secretary Chuck Hagel, during his visit to Israel this week, reaffirmed that stance.

Hagel told the House Armed Services Committee on April 11 that the U.S. had "not detected the use of chemical weapons" in Syria. He said contingency plans were being made for U.S. military options in the country.

The U.S. ambassador to Israel, Daniel Shapiro, told the Tel Aviv conference Tuesday that Israel, Jordan and Turkey are all involved in the Obama administration's contingency planning over Syria.

Brun said Israel has to be "very troubled by the possibility that chemical weapons might fall into the hands of less responsible actors.

''It is certainly possible that there will be other attacks on Israel by other organizations that obtain different types of weapons,'' he said.

— With assistance from Gwen Ackerman and Amy Teibel in Jerusalem and Gopal Ratnam and John Walcott in Washington.