Officials say Washington is moving forward with a plan to provide arms to Syrian rebels, a move that prompted a positive early reaction from U.S. allies in Europe.
The decision came after White House officials said an intelligence report found conclusive evidence that Damascus used chemical weapons, including deadly sarin gas, on a small scale against Syrian rebels during the past year.
In response, Deputy National Security Adviser Ben Rhodes said Thursday that President Barack Obama decided to authorize "direct military support" to the opposition. U.S. officials later acknowledged this support would include weapons and ammunition.
British Foreign Secretary William Hague said Friday that London agrees with the U.S. assessment on chemical weapons use, and called for a "strong, determined" response from the international community.
NATO Secretary-General Anders Fogh Rasmussen also welcomed what he called the "clear U.S. statement." He said the use of chemical weapons is "completely unacceptable" and called on Syria to let the U.N. investigate the reports.
But Russia, Syria's ally, said the evidence provided by the U.S. "does not look convincing." Yuri Ushakov, an aide to Russian President Vladimir Putin, also said the expanded U.S. military aid will hamper efforts to convene a Syria peace conference.
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. The U.S. has so far only provided non-lethal aid to the rebels.
In his Thursday statement to reporters, 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.