WASHINGTON — The White House declared Thursday that U.S. intelligence indicates Syrian President Bashar Assad has twice used deadly chemical weapons in his country's fierce civil war, a provocative action that would cross President Barack Obama's "red line" for a significant military response. But the administration said the revelation won't immediately change its stance on intervening.

The information, which has been known to the administration and some members of Congress for weeks, isn't solid enough to warrant quick U.S. involvement in the 2-year-old conflict, the White House said. Officials said the assessments were made with "varying degrees of confidence" given the difficulty of information gathering in Syria, though there appeared to be little question within the intelligence community.

As recently as Tuesday, when an Israeli general added to the growing chorus that Assad had used chemical weapons, White House spokesman Jay Carney said the administration was continuing to monitor and investigate but had "not come to the conclusion that there has been that use."

The Syrian civil war has persisted, with an estimated 70,000 dead. Obama has so far resisted pressure, both from Congress and from within his own administration, to arm the Syrian rebels or get involved militarily. He has, however, declared the use of chemical weapons a "game changer" that would have "enormous consequences."

The White House disclosed the new intelligence Thursday in letters to two senators, but had Defense Secretary Chuck Hagel announce it to reporters traveling with him in the United Arab Emirates. The letters were sent in response to questions from senators of both parties who are pressing for more U.S. involvement, and it marked the first time the administration has publicly disclosed evidence of chemical weapons use.

"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 the letters, which were signed by Obama's legislative director, Miguel Rodriguez. He went on to write that "given the stakes involved," the U.S. was still seeking "credible and corroborated facts" before deciding how to proceed.

Two congressional officials said the administration has known for weeks – and has briefed Congress – that the CIA and other intelligence agencies have evidence of two incidents of sarin gas use.

A U.S. official said intelligence agencies have had indications of chemical weapons use since March and reached the conclusions made public Thursday about two weeks ago. The two incidents are believed to have occurred around March 19 in the Syrian city of Aleppo and suburbs of Damascus, the official said.

The officials commented only on condition of anonymity because they were not authorized to speak publicly by name.

The White House described the attacks as "small scale," but the full extent of the chemical weapons use and resulting casualties was not immediately known.

Even as Assad has ratcheted up the attacks on his own people, Obama has limited U.S. assistance to non-lethal aid, including military-style equipment such as body armor and night vision goggles. However, he has repeatedly said that the use of chemical weapons, or the transfer of the stockpiles to a terrorist organization, would change things.

"That's a red line for us," he said in August. "There would be enormous consequences if we start seeing movement on the chemical weapons front, or the use of chemical weapons. That would change my calculations significantly."

A senior defense official said the White House letters were 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.

Sen. Dick Durbin of Illinois, a member of the Democratic leadership, was asked what should be done about Assad crossing the "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."

Other lawmakers questioned whether a cautious U.S. response to the newly disclosed intelligence would only strengthen Assad's resolve to keep a grip on power.

"If Assad sees any equivocation on the red line, it will embolden his regime," said House Speaker John Boehner, R-Ohio.

The White House disclosure put the U.S. in line with Britain, France, Israel and Qatar, key allies who have cited evidence of chemical weapons use. The four countries have also been pressing for a more robust response to the conflict.

U.S. commanders have laid out a range of possible options for military involvement in Syria, including establishing a "no-fly zone" or secured area within Syria where citizens could be protected, launching airstrikes by drones and fighter jets or even sending in tens of thousands of ground forces to secure the chemical weapons caches. But the military has made it clear that any action would likely be either with NATO backing or with a coalition of nations similar to what was done in Libya in 2011.

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."

Ahmad Ramadan, a member of the Syrian National Coalition opposition group's executive body, called the U.S. assertion an "important step," and he said that America had a "moral duty" to follow it with action.

The White House said the current intelligence assessments of sarin use are based in part on "physiological samples." U.S. officials said that could include human tissue, blood or other body materials, in addition to soil samples.

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.

The White House said it was still seeking to confirm the "chain of command" that led to the chemical weapons use. But officials said they were confident attacks were initiated by the Assad government, not rebels, given that they see no evidence of Assad losing control of the stockpiles.

The U.S. said the completion of a stalled U.N. investigation would be critical in confirming the use. 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.

Officials said the U.S. was consulting with allies and looking for other ways to confirm the intelligence assessments.

___

AP National Security Writers Robert Burns in Abu Dhabi, United Arab Emirates and Lara Jakes in Washington, as well as AP Intelligence Writer Kimberly Dozier, and AP writers Lolita C. Baldor and Lauran Neergaard in Washington and Zeina Karam in Beirut contributed to this report.

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