A senior administration official acknowledged that any use of chemical weapons in Syria would cross the "red line" declared by Obama many times in recent months in warnings to 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. 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."
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 White House made clear that it was resisting congressional and international calls to arm the Syrian rebels or take direct military action against Assad's forces.
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 Assad's government has refused to sign. The administration said 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 extremist group fighting alongside Syrian troops.
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, 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 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.
U.S. officials invoked the mistakes of the Iraq war as they urged caution. The administration of George W. Bush deposed Saddam Hussein in 2003, based on notoriously erroneous intelligence that he possessed weapons of mass destruction.
"Don't take from this that this is an automatic trigger," said a senior defense official, speaking on the condition of anonymity to discuss internal deliberations. "We have seen very bad movies before when intelligence is perceived to have driven policy decisions that, in the cold light of day, have been proven wrong."
The administration made no evidence in public Thursday that chemical weapons have been used, saying only that its conclusion was partly based on "physiological" data. That presumably means blood-sample analyses of victims, dead or alive, from at least one of four suspected instances of chemical-weapons use since December.
Calls for tougher U.S. action have grown in recent weeks as claims that Assad loyalists have used chemical weapons have increased.