WASHINGTON/RIYADH - While President Barack Obama has declared a "red line" over Syrian use of chemical weapons, US officials suggested on Tuesday that Washington was unlikely to respond without clear-cut evidence of such use - evidence that may be very hard to come by.

Israel's top military intelligence analyst said in Tel Aviv on Tuesday that Syrian government forces had used chemical weapons - probably the nerve gas sarin - in their fight against rebels trying to force out President Bashar al-Assad.

He cited photographic evidence of victims foaming at the mouth, their pupils contracted.

The Israeli allegations, which came during a week-long visit by Defense Secretary Chuck Hagel to the Middle East, followed similar concerns of chemical weapons use voiced by Britain and France.

But so far, those assessments appear to lack the concrete proof Washington would need to accept the kind of deeper US involvement in Syria's civil war that Obama has resisted. That, in turn, raises questions about just how well-defined the president's "red line" is.

White House spokesman Jay Carney walked a cautious line speaking to reporters, making clear that Washington was taking the Israeli accusations seriously but would require "conclusive evidence" before deciding whether to move forward.

"We have not come to the conclusion that there has been that use," Carney said. "But it is something that is of great concern to us, to our partners, and, obviously, unacceptable as the president made clear."

A senior US defense official, speaking on condition of anonymity, said that "low confidence" assessments by foreign governments could not be the basis for US action.

Officials appeared to play down the extent of any evidence of chemical weapons use provided by British and French diplomats in a letter to UN Secretary-General Ban Ki-moon's office on March 21. An Obama administration official noted it was based on second-hand sources and third-party information.

"The letter did not provide conclusive evidence of chemical weapons use, but did request a UN investigation into all allegations of use in Syria," the defense official said.

A UN team of specialists has been prevented from going to Syria to investigate the claims because of a dispute with the Damascus government over access.