Washington has so far declined to intervene in the crisis, saying doing so could worsen the conflict.
On Monday, U.S. officials said the White House and its allies were weighing military options to secure Syria's chemical and biological weapons.
Syria is believed to have several hundred ballistic surface-to-surface missiles capable of carrying chemical warheads, and a U.S. defense official said American and allied intelligence officials have detected activity around more than one of Syria's chemical weapons sites in the last week.
As the battles rage on the ground, there was growing speculation about the fate of a top Syrian spokesman who has become a prominent face of the regime.
Lebanese security officials say Jihad Makdissi, the Foreign Ministry's spokesman, flew Monday from Beirut to London. But it was not clear whether Makdissi had defected, quit his post, or been forced out. Syria had no official comment on Makdissi, who speaks fluent English and has defended the regime's crackdown on dissent.