WASHINGTON - The United States has conclusive evidence that Syrian President Bashar Assad's regime has used chemical weapons against opposition forces seeking to overthrow the government, crossing what President Barack Obama has called a "red line" that would trigger greater American involvement in the crisis, the White House said Thursday.
Officials said Obama was considering both political and military options, but it was unclear how quickly new actions would be taken and what they would involve.
"We've prepared for many contingencies in Syria," said Ben Rhodes, Obama's deputy national security adviser. "We are going to make decisions on further actions on our own timeline."
The White House said the Assad regime had used chemical weapons, including the nerve agent sarin, on a small scale multiple times in the last year. Up to 150 people have been killed in those attacks, the White House said, constituting a small percentage of the 93,000 people killed in Syria over the last two years.
The Obama administration announced in April that it had "varying degrees of confidence" that sarin had been used in Syria. But they said at the time that they had not been able to determine who was responsible for deploying the gas.
The more conclusive findings announced Thursday were aided by evidence sent to the United States by France, which along with Britain, announced it had determined that Assad's government had used chemical weapons in the two-year conflict.
Obama has said repeatedly that the use of chemical weapons would cross a "red line" and constitute a "game changer" for U.S. policy on Syria, which until now has focused entirely on providing the opposition with nonlethal assistance and humanitarian aid.
The White House said Congress has been notified of the new U.S. chemical weapons determination, as have international allies. Obama will discuss the assessments, along with broader problems in Syria, next week during the G-8 summit in Northern Ireland.
Obama is also expected to press Russian President Vladimir Putin, Assad's most powerful backers, to drop his political and military support for the Syrian government.
"We believe that Russia and all members of the international community should be concerned about the use of chemical weapons," Rhodes said.