WASHINGTON -- The Obama administration said Thursday that the Syrian government has likely used chemical weapons on a small scale against its own people, but it stopped short of threatening military action against President Bashar Assad.
In a letter to key lawmakers, the White House said U.S. intelligence agencies "assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin."
Despite the caveats, the disclosure puts President Barack Obama under new pressure to respond because it is the first time that the United States has joined other countries in suggesting that the Assad government is likely to have deployed chemical weapons over the course of the 2-year-old Syrian civil war.
A senior administration official acknowledged that any use of chemical weapons in Syria would cross the "red line" declared by Mr. Obama many times in recent months in warnings to Mr. Assad. The official, who spoke to reporters on the condition of anonymity to be candid, said the administration was waiting for a "definitive judgment."
Instead of outlining specific action, the administration reaffirmed its support for a comprehensive U.N. investigation inside Syria to gather concrete evidence. Mr. Assad has refused to admit the U.N. team amid a dispute over the scope of the investigation.
The U.S. disclosure brought a swift response from Congress, particularly from members who have argued for deeper involvement on the side of the rebels. Sen. John McCain, R-Ariz., said it was "pretty obvious that a red line has been crossed." House Speaker John Boehner, R-Ohio, accused the administration of outsourcing national security to the United Nations.
Senior Democrats also voiced concerns. Senate Foreign Relations Committee chairman Robert Menendez, D-N.J., said the Syrian government had "crossed a red line by using chemical weapons, which forces us to consider all options as to how we act to influence the balance of the conflict."
The U.S. conclusions echoed those of Britain, France and Israel, which have suggested in recent days that forces loyal to Mr. Assad have probably used sarin. The U.S. assessment, which was compiled from many intelligence agencies and finalized in recent days, eased the official skepticism that greeted the Israeli assertions just two days earlier.
The White House made clear, however, that it is resisting congressional and international calls to arm the Syrian rebels or take direct military action against Mr. Assad's forces.
"The United States and the international community have a number of potential responses available, and no option is off the table," said the White House letter, which was addressed to Mr. McCain and other lawmakers who had sought an administration response to Israel's claims.
Defense Secretary Chuck Hagel, who was traveling in the Middle East, was the first U.S. official to describe the new findings. He did not say how the administration would respond but noted, "My job is to give the president options. ... We'll be prepared to do that."
Syria possesses one of the world's largest inventories of chemical weapons, including sarin and other nerve agents banned by an international treaty that Mr. Assad's government has refused to sign. The administration said Mr. Assad remains in control of the weapons, but U.S. officials have expressed concerns that the lethal material could fall into the hands of extremists within the Syrian opposition or Hezbollah, the Lebanese Shiite militant group fighting alongside Syrian troops.
Pentagon officials have said that it could take tens of thousands of U.S. troops to secure Syria's chemical weapons as long as the civil war is raging. If the U.S. military intervened in Syria, it would almost certainly face attacks from Mr. Assad's forces. Rebel fighters allied with al-Qaida also would pose a threat.
Bombing Mr. Assad's chemical weapons stockpile could be even riskier, military analysts said. Airstrikes could easily backfire by dispersing nerve gases and other chemicals over populated areas.
Last week, the Pentagon announced that additional U.S. troops would go to Jordan to help cope with a flood of refugees crossing the border from Syria, but also to plan for possible responses to any outbreak of chemical warfare. The new troops will bring the U.S. total to about 200.
The civil war in Syria has killed more than 70,000 people and made refugees of more than 1 million Syrians. Despite the humanitarian toll, Mr. Obama has been wary of deepening U.S. military involvement. The administration, however, has widened defensive and humanitarian support for the Syrian rebels.
The main Syrian rebel group criticized what it described as a tepid U.S. response to Mr. Assad's use of chemical weapons.
" 'Small scale? Varying degrees of confidence?' The leaders of the Free Syrian Army are certain that chemical weapons are being used in Syria, so we find this whole statement odd," said Musab Abu Qatada, a spokesman for the Damascus military council, which is part of the FSA.