WASHINGTON - Despite rising calls for some kind of increased U.S. military involvement in Syria, scant evidence exists, at least in public, that Syria's vicious civil war has breached President Obama's "red line" on the use of chemical weapons.

In the 10 days since the Obama administration notified Congress that it suspected, with "varying degrees of confidence," that chemical weapons had been employed in Syria, no concrete proof has emerged, and some headline-grabbing claims have been discredited or contested. Officials worldwide now admit that no allegations rise to the level of certainty.

On Monday, Senate Foreign Relations Committee Chairman Robert Menendez (D., N.J.) alluded to chemical weapons as he proposed a measure that would provide limited arms to the rebels, asserting that President Bashar Assad's regime "has crossed a red line that forces us to consider all options."

That assertion, however, appears far less certain than it did only a week ago. British, French, and Israeli experts who expressed more confidence in their assessment than the Obama administration had in its judgment have qualified their positions in recent days, said Greg Thielmann, a former State Department intelligence analyst now with the Arms Control Association, a private organization that provides analysis of weapons issues.

Existing evidence casts more doubt on claims of use of chemical weapons than it does to help build a case that one or both sides of the conflict have employed them.

British officials now doubt the value of the few samples they have analyzed because of questions over how they were gathered, handled, and preserved.

The British defense secretary, Philip Hammond, told reporters in Washington last week that while evidence Britain had obtained led experts to suspect the use of sarin, a potent nerve gas, the samples were too degraded to be considered conclusive.

"We need hard evidence. The kind of evidence that would be admissible in court," he told a briefing of defense reporters at the British Embassy. "For that evidence to have any chance of being admitted in court, it would need to have been collected under controlled conditions, secured through a documented chain of custody to the point where it was tested. We do not yet have samples that meet that standard of evidence."

Over the weekend, Turkish doctors also cast doubt on another reported chemical attack, this one in the Syrian city of Saraqib, where rebels claimed some sort of chemical weapon had been dropped from helicopters.

But the doctors told the website Global Post that none of the blood they drew from alleged victims of the attack tested positive for nerve gas. The samples were sent to Ankara, Turkey's capital, for additional tests.

Adding to the confusion over the weekend was Carla del Ponte, a member of the United Nations' Independent International Commission of Inquiry for Syria and a former war crimes prosecutor. Del Ponte told Swiss television on Sunday that "according to the testimonies we have gathered, the rebels have used chemical weapons, making use of sarin gas."

On Monday, her Geneva-based team, which is investigating war crimes and other human-rights violations in Syria, issued a statement that emphasized that it "has not reached conclusive findings as to the use of chemical weapons in Syria by any parties to the conflict."