The Obama administration said Thursday that the Syrian government is likely to have used chemical weapons on a small scale against its own people, but it stopped short of threatening military action against President Bashar al-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 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 Syria’s two-year-old conflict.

A senior administration official reaffirmed that any use of chemical weapons in Syria would cross the “red line” described 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 reiterated its support for a comprehensive U.N. investigation inside Syria to gather concrete evidence. Assad has refused to allow the U.N. team into the country amid a dispute over the scope of the investigation.

Interactive Grid: Keeping track of the conflict in Syria through videos, images and tweets.

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 A. 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 Assad have probably used sarin. The U.S. assessment, which was compiled from many intelligence agencies and finalized in recent days, marks an easing of 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 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 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.”

Faced with difficult choices

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 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 Assad’s forces. Rebel fighters allied with al-Qaeda also would pose a threat.

Bombing 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 turned more than 1 million Syrians into refugees. Despite the humanitarian toll, Obama has been wary of U.S. military involvement. The administration, however, has widened defensive and humanitarian support for the Syrian rebels.

U.S. caution irks rebels

The main Syrian rebel group criticized what it described as a tepid U.S. assessment of the chemical weapons claims.

“ ‘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 Free Syrian Army.

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 provided no evidence in public Thursday that chemical weapons have been used in Syria, saying only that its conclusion was partly based on “physiological” data. That presumably means analyses of blood samples from victims, dead or alive, of 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. Britain and France sent letters to U.N. Secretary General Ban Ki-moon this month saying they had credible evidence that chemical weapons had been deployed.

According to senior diplomats and officials briefed on the British and French accounts, the evidence included soil samples and witness interviews that point toward the use of nerve agents in and around the cities of Aleppo, Homs and Damascus.

On Tuesday, two senior Israeli military officials said research concluded that Assad’s forces had used chemical weapons on several occasions to kill dozens of rebel fighters. They said the evidence made them “nearly 100 percent certain.”

Whitlock reported from Abu Dhabi, United Arab Emirates. Abigail Hauslohner and Ahmed Ramadan in Beirut, Colum Lynch at the United Nations and Greg Miller in Washington contributed to this report.