However, officials also said more definitive proof was needed and the U.S. was not ready to escalate its involvement in Syria beyond non-lethal aid, despite President Barack Obama's repeated public assertions that Syria's use of chemical weapons, or the transfer of its stockpiles to a terrorist group, would cross a "red line."
The White House disclosed the new intelligence Thursday in letters to two senators, and Defense Secretary Chuck Hagel, traveling in Abu Dhabi, also discussed it with reporters. The letters were sent in response to questions from members of Congress who are eager for the administration to arm the rebels or get involved militarily.
The Syrian civil war has dragged on for more than two years, with an estimated 70,000 dead. In addition to members of Congress, Leon Panetta and Hillary Rodham Clinton, as secretaries of defense and state, have urged Obama to increase U.S. involvement.
"Our intelligence community does 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," the White House said in its letters, which were signed by Obama's legislative director, Miguel Rodriguez.
Shortly after the letters were made public, Secretary of State John Kerry said on Capitol Hill that there were two instances of chemical weapons use.
It was not immediately clear what quantity of weapons might have been used, or when or what casualties might have resulted. Hagel said many of those details were classified.
Sarin is an odorless nerve agent that can be used as a gas or a liquid, poisoning people when they breathe it, absorb it through their skin or eyes, or take it in through food or water. In large doses, sarin can cause convulsions, paralysis and death. According to the Centers for Disease Control and Prevention, people usually recover from small doses, which may cause confusion, drooling, excessive sweating, nausea and vomiting.
The Aum Shinrikyo cult used sarin in attacks in the Tokyo subway system in 1995 that killed 12 people and sickened thousands.
Obama has said the use of chemical weapons would be a "game-changer" in the U.S. position on intervening in the Syrian civil war, and the letter to Congress reiterated that the use or transfer of such weapons in Syria was a "red line for the United States." However, officials quickly made clear that a stepped up U.S. response was not imminent.
A senior defense official said the White House letter was not an "automatic trigger" for policy decisions on the use of military force. The official alluded to past instances of policy decisions that were based on what turned out to be flawed intelligence, such as the Bush administration's decision to invade Iraq after concluding that Saddam Hussein had weapons of mass destruction, including nuclear weapons.
Lawmakers from both parties sounded less than patient in this case.
Sen. Dick Durbin of Illinois, a member of the Democratic leadership, was asked what should be done about the crossing of what the administration has called a red line. He said, "That's up to the commander in chief, but something has to be done."
And Sen. John McCain, R-Ariz., said, "I think it's pretty obvious that that red line has been crossed. Now I hope the administration will consider what we have been recommending now for over two years of this bloodletting and massacre and that is to provide a safe area for the opposition to operate, to establish a no-fly zone and provide weapons to people in the resistance who we trust."
The White House said the current intelligence assessments are based in part on "physiological samples." Officials wouldn't say specifically what information they are lacking in order to conclusively determine that Syrian President Bashar Assad's government used chemical weapons.
However, the White House letters emphasized a need for the completion of a stalled U.N. investigation.
But it's unclear whether U.N. inspectors will ever be able to conduct a full investigation in areas where there is the most evidence of chemical weapons use. The Syrian government has so far refused to allow the U.N. experts to go anywhere but Khan al-Assal, where Assad's government maintains the rebels used the deadly agents.
A senior administration official said the U.S. was consulting with allies and looking for other ways to confirm the intelligence assessments.
The officials commented only on condition of anonymity because they were not authorized to speak publicly.
Until Thursday, the U.S. had resisted joining a growing number of allied nations that claim to have evidence that Syrian leader Assad's government has deployed chemical weapons.
Last month, British and French ambassadors to the United Nations told Secretary General Ban Ki-moon that the government used chemical weapons near Aleppo, in Homs and possibly in the capital of Damascus. Pressure mounted on the U.S. this week when two key allies in the Middle East - Israel and Qatar - also said there was evidence that Assad had used chemical weapons.
Following the U.S. disclosure, NATO spokeswoman Oana Lungescu said, "There would doubtlessly be a very strong reaction from the international community if there were evidence that chemical weapons had been used."
The White House, in its letters to Capitol Hill, said that "because the president takes this issue so seriously, we have an obligation to fully investigate any and all evidence of chemical weapons use within Syria."
The letters were sent to McCain and to Sen. Carl Levin, D-Mich.
The letters also said the U.S. believes the use of chemical weapons "originated with the Assad regime." That is consistent with the Obama administration's assertion that the Syrian rebels do not have access to the country's stockpiles.
Associated Press writers Donna Cassata, Lara Jakes, Bradley Klapper and Lauran Neergaard contributed to this report. Burns reported from Abu Dhabi, United Arab Emirates.