The Obama administration has closed the US Embassy in Damascus and pulled all American diplomats out of Syria.
Officials say Ambassador Robert Ford and other diplomats left Syria on Monday. It is the most dramatic US move so far after 11 months of a violent crackdown on dissent by President Bashar Assad's regime.
The State Department warned last month it would close the embassy unless Assad's government stepped up its protection. It cited concerns about the safety of personnel and recent car bombs.
The UN says Assad's crackdown has killed more than 5,400 people since March. The revolt began with mostly peaceful protests, but armed rebels are now increasingly fighting the regime.
The Obama administration has long demanded that Assad step down. Officials insist his regime's demise is inevitable.
The United Kingdom too has recalled its ambassador from Syria for consultations on the escalating violence in the country, says foreign secretary William Hague.
Hague told lawmakers Monday that Britain is using multiple channels to express its "abhorrence" at the violent crackdown on dissent by President Bashar Assad's regime, and has summoned Syria's ambassador to the Foreign Office to convey that message.
His remarks came after the US said it has closed its embassy in Damascus and pulled all American diplomats out of Syria.
The Assad regime's crackdown on anti-government protesters has left thousands dead over the past year.