Supporters of Syrian President Bashar Assad threw stones and tomatoes at U.S. ambassador Robert Ford and other U.S. diplomats, and besieged the building of an opposition figure in Damascus whom the diplomats were visiting on Thursday, a witness and diplomatic sources said.

"Two embassy cars were damaged. The U.S. delegation is still there and the crowd is surrounding the building," said the witness.

The witness said the diplomats were visiting Hassan Abdelazim, a centrist politician who has been demanding an end to a crackdown on a six month pro-democracy uprising as a condition for any talks with Assad.

According to UN estimates, 2,700 people have been killed in Syria during the uprising.

Also on Thursday, Syria accused the U.S. of inciting violence against its security forces and said it would confront what it described as attempts to interfere in its domestic affairs.

"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," a Foreign Ministry statement said.