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 Barack 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.
Yet political rhetoric on Syria has overtaken actual evidence in a high-stakes Washington debate that is increasing pressure on Mr. Obama to lend more military support to rebels fighting to topple Syrian President Bashar Assad.
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 Mr. Assad's regime "has crossed a red line that forces us to consider all options."
But that assertion 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 in recent days qualified their positions, said Greg Thielmann, a former State Department intelligence analyst now with the Arms Control Association, a private organization that provides analysis of weapons issues.
"That should make everyone suspicious," Mr. Thielmann said. "And the reality may be a lot more complicated." He added, "Do you really risk going to war without knowing who has used what, and in what circumstances?"
Existing evidence casts more doubt on claims of chemical weapons use 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. 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.
Turkish doctors over the weekend 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 Global Post website that none of the blood they drew from alleged victims of the attack, who had been taken to Antakya, Turkey, for treatment, tested positive for nerve gas. Samples were sent to Ankara, Turkey's capital, for more 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. Ms. Del Ponte told Swiss television Sunday, "According to the testimonies we have gathered, the rebels have used chemical weapons, making use of sarin gas." On Monday, her Geneva-based team, investigating war crimes and other human rights violations in Syria, issued a statement emphasizing "that it has not reached conclusive findings as to the use of chemical weapons in Syria by any parties to the conflict."
Conclusive testing to prove or disprove chemical weapons use would require an impartial body -- most likely the United Nations, with the help of the Organization for the Prohibition of Chemical Weapons -- to be on the ground to take soil and blood samples. Mr. Assad's regime -- claiming much of the international community would not treat it fairly -- has resisted the idea of a U.N. team unless it included experts from Russia, which continues to support the Syrian regime.
Without such proof, White House spokesman Jay Carney urged caution Monday against taking "precipitous action" based on limited evidence. He called intelligence assessments "extremely valuable and significant," but insufficient to make a final determination on whether chemical arms had been used.
"We are now in the process of gathering the facts, not rushing to conclusions, not acting precipitously based on an incomplete case, but gathering the facts in order to make a judgment about what policy actions the president might take in reaction to the crossing of the red line," he said.
Still, Mr. Carney was dismissive of Ms. Del Ponte's suggestion that it was the rebels, not the government, who had used chemical weapons, describing the White House as "highly skeptical" of the idea. A State Department official said rebels aren't believed to possess chemical arms, and Pentagon spokesman George Little said that "if chemical weapons were used, the Syrian regime would be responsible."
"Facts are not complete, as the president himself has said, and we need to continue to gather facts and to get facts that are corroborated, and so we are doing that both through our own means and working with partners," a senior State Department official told reporters Monday, speaking on condition of anonymity to discuss the matter candidly.
Meanwhile in Syria, more than 42 Assad regime soldiers were killed and 100 others remained unaccounted for in reported Israeli airstrikes Sunday outside Damascus, said the opposition group Syrian Observatory for Human Rights on Monday, providing the first unofficial accounting of casualties in attacks that raised concerns about an escalation of the conflict. In Israel, officials continued to predict that Syrian retaliation was unlikely, as Israelis debated how much further their forces could go without sparking an unwanted war with Syria or its allies.
The casualty estimates came from the British-based Syrian Observatory for Human Rights, a pro-opposition monitoring organization. Syrian authorities have released no official casualty figures from the pre-dawn bombardment outside the capital.
The strikes were believed to be the second Israeli attack on Syria in recent days, following a reported airstrike Friday.
Israel has not confirmed the attacks, but officials have indicated privately that they were intended to destroy sophisticated missiles destined for Hezbollah, the militant Lebanese group closely allied with Syria and Iran. Israeli officials stressed that their nation was not taking sides in Syria's civil war and acted strictly to disrupt the Hezbollah arms pipeline from Syria.
Syria called the Israeli explanation a pretext and denied that arms were being transferred outside its borders. Syria said it would respond in time to what it called an unprovoked act of war.
Los Angeles Times contributed.