The White House says it now has conclusive evidence Syria's government has used chemical weapons against rebels, prompting it to authorize direct military support to the opposition.
Deputy National Security Adviser Ben Rhodes said the U.S. has "high confidence" Damascus used chemical weapons, including the nerve agent sarin, on a small scale multiple times in the last year, resulting in as many as 150 deaths.
In response, Rhodes said President Barack Obama has decided to send "direct military support" to the rebel fighters. He declined to comment on the nature of the support, saying only it would differ in "both scope and scale" compared to current U.S. aid.
But U.S. officials, speaking anonymously, later acknowledged weapons and ammunition would be included in the aid package to the rebels. So far, the U.S. has only provided the Syrian rebel army with rations, medical supplies and other non-lethal aid.
U.S. intelligence officials have been saying for months they suspect chemical weapons were used by the Syrian government . But Mr. Obama had said he needed to see firm evidence before deciding his next move.
Rhodes stressed that Mr. Obama views the use of chemical weapons as a "red line" that would prompt greater U.S. involvement in the Syrian conflict. And he said the latest news has changed his "calculus."
But he cautioned that the White House still does not support sending American troops to Syria, and that no decision has been made on other military options, such as the enforcement of a no-fly zone.
The announcement follows a week of White House negotiations that have revealed a deep division among senior administration officials regarding the U.S. role in Syria's civil war.
The president has also come under increasing pressure from lawmakers and others, including former President Bill Clinton, to take more forceful action in Syria.
The debate took on an even greater sense of urgency on Thursday, after the United Nations announced it has confirmed that nearly 93,000 people have been killed in Syria over the last two years.
President Obama has been reluctant to arm the Syrian rebels, out of concern the weapons may end up in the hands of Islamic extremists that make up a growing part of the opposition forces fighting to overthrow President Bashar al-Assad.
Mr. Obama's Thursday decision comes after government forces dealt the opposition a series of demoralizing defeats in recent weeks, including the capture of the strategic town of Qusair and winning the support of Lebanese Hezbollah fighters.