The State Department advised those who remain in Syria to restrict their movements, as the Syrian government intensified a violent crackdown on opposition protesters.
The warning came as congressional calls grew for the Obama administration to impose severe new sanctions on President Bashar Al-Assad's regime.
In a new travel warning, the department said Americans should depart Syria while commercial flights and other transportation are still available "given the ongoing uncertainty and volatility of the situation." It noted that Syrian authorities had imposed tight restrictions on the ability of US and other diplomats to move around the country.
The advisory echoed a warning issued in late April when the department ordered the non-essential personnel and the families of all American staff at the US Embassy in Damascus to leave the country. But it came as Al-Assad's regime has stepped up efforts to quell the uprising with military force, particularly in the opposition stronghold of Hama, where as many as 250 people have been killed since Sunday.
The crackdown has drawn widespread international condemnation and the Obama administration has said that Assad has lost his legitimacy and that Syria would be a better place without him in power. On Thursday, Secretary of State Hillary Rodham Clinton said the US believes that the regime has killed more than 2,000 people in trying to put down the unrest that is now in its fifth month.
Congress, meanwhile, urged President Barack Obama to impose tougher sanctions on Al-Assad's regime. The Treasury Department on Thursday added an Al-Assad confidante and his firm to a US terrorism blacklist, which already includes the president and some of his inner circle.
In a letter to Obama sent on Friday, 221 members of the House from both parties called on the administration to bar all US businesses from operating in Syria and to block all property transactions with the Syrian government that could be subject to US jurisdiction.
"The threat posed by the Assad regime to the US, to our allies, and, most of all, to the Syrian people is stark and growing," wrote the chairman and ranking member of the House Foreign Affairs Committee, Ileana Ros-Lehtinen and Howard Berman. "Assad and his circle have no legitimacy to rule." "The US and all responsible nations must impose the strongest possible sanctions on the murderous Syrian regime," they said.
Administration officials have said they are moving to impose sanctions on Syria's energy sector, but none have yet been forthcoming.