Secretary of State John Kerry and spokesmen from the White House and the Pentagon said investigations haven’t produced hard proof that Bashar al-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 Brigadier General Itai Brun, chief of research and analysis for the army’s military intelligence division, cited photographic and other forms of evidence today 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.

‘Apparently Sarin’

“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.

Still, a senior researcher at the Tel Aviv institute expressed doubt about Brun’s assertion.

“I know how it looks when chemical weapons attacks take place and what is happening in Syria doesn’t look like that,” said Shlomo Brom, a retired general and former chief of the Israeli army’s strategic planning division. “The number of casualties is too low and the sources of information are not credible.”

The evidence cited by Israeli military intelligence “can be orchestrated,” Brom said. “It’s quite clear that the rebels have a strong interest to show the world because that will bring international intervention, which is what they want.”

Conclusive Evidence

U.S. Secretary of State John Kerry told reporters that Israeli Prime Minister Benjamin Netanyahu had been unable to confirm the reports in a phone call today.

“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 U.S. will work with allies to “monitor, investigate and verify any credible allegations,” Little said.

Crowd Control

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 in 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-Qaeda or Iranian ally Hezbollah could inherit some or all of Assad’s chemical and biological weapons.

The U.K. 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.

Changing Calculations

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 today 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.