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

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

Ford has come in for heavy criticism by regime supporters inDamascus 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.

The US State Department said Ford was brought back to Washington because of "credible threats against his personal safety in Syria."

"At this point, we can't say when he will return to Syria," deputy spokesman Mark Toner said in Washington. "It will depend on our assessment of Syrian regime-led incitement and the security situation on the ground."

The spokesman said Washington hoped the Syrian government would end its "incitement campaign" against the ambassador.

"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."

The US Senate earlier this month unanimously confirmed Ford's appointment as ambassador to Syria, calling it "a tough message" to Assad and a sign of US "solidarity with the Syrian people."

"Despite even being physically attacked and assaulted by the regime's goons, Ford continues courageously to visit cities under military siege and speak truth to power," US Senator John Kerry said at the time.

Washington has called on the UN Security Council and the international community to "dramatically" increase pressure on Syria over its bloody response to opposition to the government in Damascus.

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.

Assad's supporters late last month tried to attack Ford and embassy staff as they visited a Syrian opposition leader in Damascus.

Secretary of State Hillary Clinton demanded in the wake of the attack that Syria "take every possible step" to protect US diplomats.

Toner said at the time the pro-regime demonstrators seriously damaged US vehicles and pelted the visitors with tomatoes but did not hurt Ford or his staff.

Following the incident, the United States summoned the Syrian ambassador to Washington, Imad Mustapha, and according to State Department spokeswoman Victoria Nuland, he was "read the riot act."

Mustapha "was reminded that Ambassador Ford is the personal representative of the president (Barack Obama) and an attack on Ford is an attack on the United States," Nuland said.

"He was also asked for compensation for our damaged vehicles," she said.