Officials say the United States is proceeding with a plan to arm 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 on a small scale, including deadly sarin gas, against Syrian rebels during the past year.
On Friday, Syria dismissed as "lies" the U.S. claims it had used chemical weapons against the rebels. Syria's Foreign Ministry said the U.S. accusation was based on "fabricated information."
British Foreign Secretary William Hague said Friday that London agrees with the U.S. assessment on chemical weapons use.
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."
Russia, Syria's ally, said the evidence provided by the U.S. "does not look convincing."
U.S. intelligence officials have been saying for months they suspect Syria's government has used chemical weapons. But Mr. Obama had said he needed to see firm evidence before deciding on his next move. The U.S. has so far provided only non-lethal aid to the rebels.
On Thursday, U.S. Deputy National Security Adviser Ben Rhodes said President Obama decided to authorize direct military support to the opposition. U.S. officials later acknowledged this support would include weapons and ammunition.
Rhodes also stressed Obama views the use of chemical weapons as a "red line" that would prompt greater U.S. involvement in the Syrian conflict.
But he cautioned the White House still does not support sending U.S. troops to Syria, and that no decision has been made on other military options, such as the enforcement of a no-fly zone.
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.