The United States has pulled its ambassador out of Syria because of what it said were credible threats to his safety after his public support for protesters led to attacks on his embassy and diplomatic convoy by supporters of President Bashar al-Assad.

Robert Ford left as a crackdown on protests and a nascent armed insurgency intensified despite Western condemnation, and more businesses and shops closed in southern Syria in the most sustained strike of the seven-month uprising.

His departure signals a further deterioration of relations between Syria's rulers and Washington, which has called for Assad to step down and deepened sanctions on Damascus to include the country's small but key oil sector, Reuters reported.