Anthony Shadid reports: The United States closed its embassy in Syria on Monday and withdrew all staff there in response to escalating mayhem in the country and what American officials called the Syrian government’s unbridled repression of an 11-month-old uprising that has become the bloodiest conflict in the Arab Spring revolts.

Clearly laying the blame on Syria’s president, Bashar al-Assad, the State Department said in a statement on its Web site that the United States had “suspended operations of our Embassy in Damascus,” and that Ambassador Robert S. Ford “and all American personnel have now departed the country.” It said the closure reflected “serious concerns that our Embassy is not protected from armed attack.”

The announcement did not signal a formal break in American diplomatic relations with Syria but was considered a strong signal that Obama administration officials believe there is nothing left to talk about with Mr. Assad. His government has been emboldened since an Arab League peace proposal for Syria appeared to collapse in a diplomatic failure over the weekend at the United Nations, where both Russia and China vetoed a Security Council resolution to endorse it.

Word of the embassy closure came as opposition groups reported that Syrian government forces shelled the battered city of Homs for another day striking a makeshift clinic and killing at least 17 people in a mounting toll that has made the city the epicenter of the uprising, which began last March. The city, Syria’s third-largest, has emerged as an arena of some of the revolt’s worst violence, with scores dead there in just the past few days.