By Matt Spetalnick and David Alexander
WASHINGTON/RIYADH (Reuters) - While President Barack Obama has declared a "red line" over Syrian use of chemical weapons, U.S. 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 U.S. 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 U.S. defense official, speaking on condition of anonymity, said that "low confidence" assessments by foreign governments could not be the basis for U.S. 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 U.N. 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 U.N. investigation into all allegations of use in Syria," the defense official said.
A U.N. team of specialists has been prevented from going to Syria to investigate the claims because of a dispute with the Damascus government over access.
MEMORIES OF IRAQ
On a visit to Israel last month, Obama said of reports the Syrian government may have used chemical weapons, "Once we've established the facts, I have made clear the use of chemical weapons is a game-changer."
The Obama administration's determination to avoid committing itself without air-tight proof, plus international backing, is due in part to the lessons of Iraq, a source close to White House policymaking said recently.
Then, the George W. Bush administration used faulty intelligence to justify the Iraq invasion in pursuit of nuclear, chemical and biological weapons that turned out not to exist.
"There will be no rush to judgment," the source said.
U.S. officials and experts have cited the difficulty for the United States in confirming chemical weapons use in Syria.
For example, officials have said they are reluctant to give much credence to information on alleged chemical weapons use that emanates from the Syrian opposition, considering such claims suspect because of a vested interest to get Washington involved militarily.
So far, the United States has limited itself to mostly non-military support for the opposition. Last weekend, Secretary of State John Kerry announced a new package of non-lethal aid partly destined to rebel fighters. That has fallen far short, however, of what some U.S. lawmakers, U.S. allies like Britain and France and Syrian opposition leaders themselves have sought.
Washington could face further criticism if it is perceived to have failed to enforce Obama's chemical weapons ultimatum to Assad, who has clung to power despite repeated U.S. calls to step down.
Raymond Zilinskas, a chemical and biological weapons expert at the Monterey Institute's James Martin Center for Nonproliferation Studies, said photographic evidence cited by the Israeli official - with victims foaming at the mouth - showed symptoms consistent with the use of a nerve agent such as sarin gas.
But he questioned whether photographic evidence alone could prove chemical weapons use.
"The difficult part is - what you really need are samples," said Zilinskas.
Sarin or other nerve agents would linger in blood and tissue samples for some time, and probably longer in hair samples, he said. "That's almost like a smoking gun."
It is not known if Western intelligence agencies, perhaps with aid from Syrian rebels, have procured biological or soil samples from the sites of the alleged attacks last month.
Even if proof of chemical weapons use met Washington's standards of proof, U.S. action might further be delayed while intelligence analysts try to figure out how widespread it was - a factor that would determine the extent of any U.S. response.
While contingency plans have been crafted, U.S. officials have continued grappling with questions such as whether U.S. forces would be able to locate enough of Assad's stockpile and whether the material could be "safe-guarded" inside Syria in the midst of civil war or whether it would have to be taken out.
Another wild card could be how Israel might respond. Carney declined to answer a question whether the White House had been aware that Israel would go public with the accusation on Tuesday and whether it was prudent to do so.
The source close to White House policymaking speculated that Israel may have gone public with its findings to send a message to Assad that its military had Syria's chemical weapons stockpile in its sights, and would not hesitate to take action if deemed necessary to secure it.
(Writing by Phil Stewart; Additional reporting by Warren Strobel and Peter Cooney)