BEIRUT (AP) -- Supporters of President Bashar Assad stoned the convoy of the U.S. ambassador to Syria as he arrived for a meeting with a leading opposition figure on Thursday, then pelted him with eggs and tomatoes and tried to break into the building while he was inside, the opposition activist and a U.S. official said. The American envoy was trapped in the office for three hours by the angry mob outside.

Ambassador Robert Ford, an outspoken critic of Assad's crackdown on the 6-month-old anti-government uprising, was unharmed and eventually escorted out by Syrian security forces, who showed up more than an hour after the attack began. He was meeting with Hassan Abdul-Azim, who heads the outlawed Arab Socialist Democratic Union party.

"Now that security forces are here, I believe his life is not in danger," Abdul-Azim told The Associated Press.

Ford has angered the Syrian regime in past months by visiting a couple of the protest centers outside of Damascus in a show of solidarity with the anti-government uprising. The latest incident could further raise tensions between Washington and Damascus, which has accused the United States of helping incite violence in Syria. In August, President Barack Obama demanded Assad resign, saying he had lost his legitimacy as a ruler.

"A crowd of demonstrators tried to assault Ambassador Ford and embassy colleagues today as they went about doing the normal work of any embassy," U.S. State Department spokesman Mark Toner said in Washington. He said the ambassador went to a meeting with "a well-known Syrian political figure," adding that Ford and his staff were back safe at the U.S. Embassy.

"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," said Toner. "Syrian security officers finally assisted in securing a path from the ambassador's meeting for him and his aides back to the embassy."

Syria's Foreign Ministry said the U.S. Embassy informed the ministry that Ford was confronted by protesters when he visited Abdul-Azim. The statement added that the ministry immediately contacted security authorities, who "took all measures needed to protect the ambassador and his team and secured their return to their work in accordance with Syria's international commitments."

Abdul-Azim said Ford was inside his office when the Assad supporters tried to force their way in, breaking some door locks. Office staff prevented them from rushing in, but the ambassador was trapped inside for about three hours by the hostile pro-government protesters outside.

University student Majd Mutlaq, 21, stood outside Abdul-Azim's office with a bag of eggs and tomatoes, saying he came after he heard the ambassador was inside the building.

"We don't want him anywhere in Syria and if I ever see him, I will throw at him whatever I am carrying," he said.

The attack on Ford came five days after government supporters threw eggs and stones at France's ambassador as he left a meeting in Damascus with a Greek Orthodox patriarch. Ambassador Eric Chevallier was unharmed.

Tension between the West and Syria - Iran's closest Arab ally - have been rising for months.

Washington and the European Union have imposed sanctions on some Syrian officials, including Assad, because of Assad's crackdown that has left some 2,700 people, according to the United Nations.

A trip in July by the U.S. and French ambassadors to the central city of Hama to express support for protesters drew swift condemnation from the Syrian government, which said the unauthorized visits were proof that Washington was inciting violence in the Arab nation. Authorities then warned both ambassadors not to travel outside the capital without permission.

A month later, the Obama administration brushed off a complaint by Syrian authorities that Ford violated their travel rules by leaving Damascus without permission. The Syrian foreign ministry registered concern over Ford's trip in August from Damascus to the southern village of Jassem, where he met opposition activists.

Last month, Ford and several other ambassadors expressed their condolences to the family of a rights advocate who was killed.

The U.S. has maintained diplomatic relations with Syria even while protesting Assad's efforts to crush the uprising.

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AP writer Matthew Lee in Washington contributed to this report.

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Bassem Mroue can be reached at http://twitter.com/bmroue

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