US Ambassador to Syria Robert Ford was brought back to Washington because of "credible threats against his personal safety in Syria," the State Department announced on Monday.
"At this point, we can't say when he will return to Syria," deputy spokesman Mark Toner said. "It will depend on our assessment of Syrian regime-led incitement and the security situation on the ground."
Ford has come in for heavy criticism by regime supporters in Damascus who have accused him of helping incite violence in the country.
The ambassador has also angered the regime by visiting protest hubs outside the capital in a show of solidarity with pro-democracy demonstrators.
According to the United Nations, more than 3,000 people, mostly civilians, have been killed in the crackdown on the protests, which, inspired by uprisings in other Arab nations, first erupted in March.
The spokesman said Washington hoped the Syrian government would end its "incitement campaign" against Ambassador Ford.
"Ambassador Ford's presence is a benefit to our mission in Syria as he has worked diligently to deliver our message and be our eyes on the ground," Toner said. "This decision was based solely on the need to ensure his safety, a matter we take extremely seriously."