US casts doubt on claims of chemical weapons in Syria US officials say they have not 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 had produced no 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."" I don't know yet what the facts are. I don't think anybody knows what they are yet. US Secretary of State John Kerry ""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."" A senior Israeli army officer said the Assad regime was using chemical weapons, most likely sarin. But US Secretary of State John Kerry told reporters that Israeli Prime Minister Benjamin Netanyahu had been unable to confirm the reports in a phone call on Tuesday. ""I talked to Prime Minister Netanyahu this morning from here,"" Mr Kerry said at a NATO gathering 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,"" Mr Kerry said. ""I don't think anybody knows what they are yet."" The differing US and Israeli assessments, one official 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 Assad's Iranian ally Hezbollah could inherit some or all of Assad's chemical and biological weapons. Britain and France have also alleged that Mr Assad's troops have used chemical weapons. Mr Obama said when he visited Israel last month that proof Syria used chemical weapons would change US calculations in dealing with the conflict, without specifying what actions he would take. US DefenCe Secretary Chuck Hagel, during his visit to Israel this week, reaffirmed that stance. General Brun made his remarks to the annual INSS security conference in Tel Aviv. He said the military had seen evidence indicating the use of chemical agents on several occasions, including in an incident on March 19. General Brun said the symptoms were observed in photographs taken of the affected area after the attacks in question. Bloomberg, AFP