AMMAN - Supporters of Syrian President Bashar al-Assad hurled rocks and tomatoes at US ambassador Robert Ford's convoy as he visited an opposition figure in Damascus on Thursday in an attack the US said was "wholly unjustified".

Ford and his party were not injured, the US State Department said, but several embassy vehicles were damaged and the ambassador had to lock himself in an office to await help from Syrian security.

Syria, which has been irked by Ford's meetings with opposition figures, accused Washington of inciting violence and meddling in its affairs. Washington demanded that Syria take steps to protect US diplomats.

"We condemn this unwarranted attack in the strongest possible terms. Ambassador (Robert) Ford and his aides were conducting normal embassy business and this attempt to intimidate our diplomats through violence is wholly unjustified," US Secretary of State Hillary Clinton said.  

"We immediately raised this incident with the Syrian government and we are demanding that they take every possible step to protect our diplomats according to their obligations under international law."

State Department spokesman Mark Toner said Ford had been in touch with senior department officials. "He is calm. He is resolute. And he is determined to continue to carry out his duties," he said.  

The diplomats were visiting Hassan Abdelazim, a centrist politician who has demanded an end to Assad's crackdown as a condition for any opposition dialogue with the president.

"Two embassy cars were damaged," said a witness, who asked not to be identified, adding that the demonstrators were chanting "Abu Hafez (father of Hafez)", a nickname for Assad.

Ford was already inside the building when about 200 Assad supporters attacked the embassy vehicles with large rocks and street signs with concrete bases. Embassy staff inside the vehicles were not harmed. Police later extracted the convoy.

The Syrian government said that once they were alerted to the confrontation, authorities "took all necessary procedures to protect the ambassador and his team and secure their return to their place of work".

The State Department said that when Ford and his aides arrived at the building housing Abdelazim's office, they were met by a "mob" of pro-government demonstrators who followed them to the office, shouting slogans. When Ford's group reached the office they shut themselves inside and called for assistance from Syrian security.

No one in the American part was injured, the State Department said, but several embassy vehicles were damaged.  

Soon after the incident, the Syrian Foreign Ministry issued a statement accusing the United States of "encouraging armed groups to practice violence against the Syrian Arab Army".

The attack was the second on US diplomats since protests erupted in March. Assad supporters assaulted the US embassy in July after Ford visited the flashpoint city of Hama.