The United States has closed its embassy in Syria and pulled remaining diplomats, citing a worsening security situation.

The State Department said Monday that embassy functions have been suspended and that Ambassador Robert Ford and other diplomats have left the country. The United States had warned last month it would close its mission in Damascus unless Syria's government addressed security concerns, citing the safety of personnel and recent car bombs.

The move, further isolating President Bashar al-Assad's government over its bloody crackdown on anti-government protesters, comes amid more assaults in Syrian cities.

Rights activists said at least 24 people were killed and many more wounded when government forces launched a sustained assault on the flashpoint city of Homs.

Arabic satellite television stations broadcast images from Homs Monday showing smoke billowing into the sky from buildings damaged in the violence. The attacks occurred in several districts of the city, where an oil pipeline also exploded. Syrian state television says “terrorist gangs” were behind the bombings.

There was no independent confirmation of the casualties as Syria restricts independent reporting in the country.

The violence comes several days after activists reported what they called a massacre in Homs, saying at least 200 people died in a government bombardment late Friday into Saturday, in what appeared to be one of the deadliest incidents of the 11-month revolt.

Homs has become a focal point of opposition resistance to the government.

China and Russia vetoed the Western and Arab-backed resolution on Saturday, a move criticized by Western governments.

Russian Foreign Minister Sergei Lavrov said Russia asked the United Nations Security Council to wait to vote on the resolution. He announced a visit to Damascus beginning on Tuesday.

Russia said it vetoed the resolution because of what it viewed as “ultimatum-like” demands for the removal of Mr. Assad. China said it issued a veto to oppose what it perceives as an effort to promote “regime change” in Syria through “external force,” in violation of international norms.

In Washington, U.S. President Barack Obama vowed to apply sanctions and increase pressure on Mr. Assad, but said the Syrian crisis could be resolved without outside military intervention.

Syria's opposition uprising against Mr. Assad's autocratic rule has escalated in recent months into open conflict between rebels and pro-Assad forces. Last month, the United Nations estimated the death toll from the unrest at 5,400 before it stopped updating the figure because of difficulties in obtaining information.