Washington: The US on Monday closed its embassy in Syria and pulled out its staff, including ambassador Robert Ford over security concerns, the State Department said.

"The US has suspended operations of our embassy in Damascus as of February 6. Ambassador Ford and all American personnel have now departed the country," State Department spokeswoman Victoria Nuland said in a statement.

At least 5,400 people have been killed in the Syrian government's 11-month crackdown on protesters, according to the UN. Syrian authorities blame the violence on armed gangs affiliated with Al Qaeda and say more than 2,000 soldiers and police have been killed.

Nuland's statement comes as the Syrian army launched a fierce assault on the city of Homs. At least 15 people are believed to have been killed till on Monday, as reported.

"The recent surge in violence, including bombings in Damascus on December 23 and January 6, has raised serious concerns that our embassy is not sufficiently protected from armed attack," Nuland said.

The Syrian government has "failed to respond adequately" to US security concerns, she added. The spokeswoman said Ford left Damascus but remains the US ambassador to Syria and its people.

"As the president's representative, he will continue his work and engagement with the Syrian people as head of our Syria team in Washington," she said.

"Together with other senior US officials, Ambassador Ford will maintain contacts with the Syrian opposition and continue our efforts to support the peaceful political transition which the Syrian people have so bravely sought," the spokeswoman said.