US envoy to Syria Robert Ford, an open critic of President Bashar Al-Assad's crackdown on political protest, has gone on "indefinite" leave for security reasons, an embassy official said on Monday.

"Ambassador Robert Ford is on leave indefinitely. Washington decided to give him the leave out of concerns about his personal safety," the official told AFP. 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.