|Assad supporters assaulted Ford's convoy in July after he visited the flashpoint city of Hama [YouTube]
Dozens of deaths have been reported in Syria, just hours after government supporters attacked the US ambassador's convoy with tomatoes and eggs as he and his colleagues met an opposition figure in Damascus.
Robert Ford, an outspoken critic of President Bashar al-Assad's crackdown on the seven-month-old anti-government uprising, was "safely back at the US embassy" following Thursday's attack, Mark Toner, deputy spokesman of the US state department, said.
The diplomats were meeting Hassan Abdul-Azim, a centrist politician who has demanded an end to the crackdown as a condition for any opposition dialogue with Assad.
Abdul-Azim said Ford was inside his office in the Syrian capital when the pro-Assad demonstrators tried to force their way in, breaking down some locked doors.
Office staff prevented them from rushing in, but Ford was trapped inside for about three hours with up to 100 hostile government supporters outside.
Abdul-Azim said security forces arrived about an hour after the attack began.
"The mob was violent; it tried, unsuccessfully, to attack embassy personnel while they were inside several embassy vehicles, seriously damaging the vehicles in the process," Toner said.
"Syrian security officers finally assisted in securing a path from the ambassador's meeting for him and his aides back to the embassy."
White House condemnation
The US administration condemned the incident, calling it "unwarranted" and "unjustifiable".
Hilary Clinton said it was "wholy unjustified", urging Syria to protect diplomatic staff.
She said the attack was "unwarranted" and condemned it "in the strongest possible terms."
"This is clearly part of a campaign to intimidate dipilomats who are bearing witness to brutality of the Assad regime," Jay Carney, the White House spokesman, said on Thursday.
He said the US will not recall Ford and urged the senate to "show its support and confirm [Ford] so he can continue his work."
In-depth coverage of escalating violence across Syria
For its part, the Syrian foreign ministry issued a statement accusing the US of "encouraging armed groups to practice violence against the Syrian Arab Army".
The statement said: "Recent statements from American administration officials ... clearly indicate that the United States is involved in encouraging armed groups to practice violence against the Syrian Arab Army."
Ford has angered Damascus in the past, notably by visiting the flashpoint city of Hama with his French counterpart in July, ignoring a new ban on Western diplomats travelling outside the Damascus area.
Assad supporters assaulted both the French and US embassies soon after the visit, winning cheers from protesters who later faced a tank-led crackdown.
Assault on Rastan
Thursday's incident in Damascus came on a day an activists' body accused security forces of killing at least 27 people, including a child, during two days of an offensive to recapture the central town of Rastan.
"The city is still under heavy shooting and shelling from the army's heavy weapons amid of news about major defections in the army trying to protect the people," the Local Co-ordination Committees said in a bulletin.
The Reuters news agency reported that two of the 27 killed were military deserters and the rest were local residents, as the town came under a tank and helicopter-backed assault.
Seventeen additional deaths were reported in Homs, Avaaz, an international rights and advocacy group, reported later on Thursday, citing reports by citizen journalists in the city.
Battles are raging between the Syrian army and defected soldiers, who have formed into what they call the "Khaled Batallion", Avaaz said in a bulletin.
The US and the European Union have imposed sanctions on some Syrian officials, including Assad, because of the security crackdown that has left some 2,700 people, according to the UN.