The United States Friday said it would close its mission in Damascus unless the Syrian government increases its physical protection of the embassy, dpa reported.
The US Department of State said that it has requested that Syria take additional security measures.
"We have also advised the Syrian government that unless concrete steps are taken in the coming days we may have no choice but to close the mission," a spokesperson said in a statement.
The department noted its serious concerns about the deteriorating situation in the capital city, "including the recent spate of car bombs," and about the security of its personnel there.
Protests were held across Syria after Friday prayers. Security forces killed 15 civilians, mainly in the central city of Homs, a hotbed of anti-government demonstrations.
Armed resistance has grown in recent months, with rebels staging increasingly sophisticated attacks against the Syrian army and more soldiers defecting to the rebel forces.
More than 5,000 people have been killed in Syria since anti-government protests erupted in March, according to the United Nations. Opposition activists says some 500 civilians have been killed by government forces since Arab observers started their mission in mid-December
Earlier Friday, the White House said that President Bashar al-Assad was losing control and his fall was "inevitable." He rejected the suggestion that the international community should launch another Libya-style intervention.
"It is clear that his regime is no longer ... in full control of the country and that it is only taking Syria toward a dangerous end," White House spokesman Jay Carney said.
The Syrian opposition on Friday urged the Arab League to refer Syria to the United Nations Security Council after a mission by the pan-Arab body submitted a report on the deadly violence. But Assad's friends on the Security Council - Russia and China - have already blocked months of efforts for a resolution.
Other calls have come for stronger, direct intervention in the situation in Syria.
Carney dismissed the suggestion that the international community has failed to get involved.
"The international community is not standing by," he said. "The international community has, with American leadership, taken action to pressure and isolate Syria, and that is having an obvious effect as the Assad regime loses control of the country."