WASHINGTON — The Obama administration said on Thursday that the Syrian government has likely used chemical weapons against its people, but it stopped short of threatening military action against President Bashar Assad.
In a letter to key lawmakers, the White House said US 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 Obama under more 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 two-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 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 UN investigation inside Syria to gather concrete evidence. Assad has refused to admit the UN team amid a dispute over the inquiry’s scope.
The US disclosure brought a swift response from Congress, particularly from members who have argued for deeper involvement on the side of the rebels. Senator John McCain, Republican of Arizona, said it was ‘‘pretty obvious that a red line has been crossed.’’
Senior Democrats also voiced concerns. Senate Foreign Relations Committee Chairman Robert Menendez of New Jersey said the Syrian government’s action “forces us to consider all options as to how we act to influence the balance of the conflict.’’
The US conclusions echoed those of Britain, France and Israel, which have suggested in recent days that forces loyal to Assad have probably used sarin. The US assessment was compiled from many intelligence agencies and finalized in recent days.
The White House made it clear, however, it is resisting congressional and international calls to arm the Syrian rebels or take direct military action against 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 McCain and other lawmakers.
Defense Secretary Chuck Hagel, who was traveling in the Middle East, was the first US 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. US officials have expressed concerns that the lethal material could fall into the hands of extremists within the Syrian opposition or Hezbollah, the militant group fighting with Syrian troops.
Pentagon officials have said that it could take tens of thousands of US troops to secure Syria’s chemical weapons as long as the civil war is raging.
US officials invoked the mistakes of the Iraq War as they urged caution. The administration of George W. Bush invaded in 2003, based on notoriously erroneous intelligence that Iraq possessed weapons of mass destruction.