Syria has accused the US of "interfering" in its affairs after the US ambassador to the country travelled to the flashpoint city of Hama.The Syrian foreign ministry said the visit by Robert Ford was "obvious proof" of US involvement in continuing protests in the country.
Earlier, the US State Department said Mr Ford's visit was to show solidarity with protesters.
Hundreds of Hama residents have fled fearing an assault by security forces.
Tanks are stationed outside the city and at least 22 people have been shot dead in recent days.
Washington says Mr Ford hopes to stay in Hama for anti-government protests which normally follow Friday prayers.
The BBC's Kim Ghattas in Washington says the ambassador's presence in Hama could dissuade Syrian President Bashar al-Assad from launching an all-out assault on the city.
But the move has angered Damascus.
"The presence of the US ambassador in Hama without previous permission is obvious proof of the implication of the United States in the ongoing events, and of their attempts to increase (tensions), which damage Syria's security and stability," the foreign ministry said in a statement.
Last Friday's protest in Hama was one of the largest in three months of demonstrations across Syria.
A day later, Mr Assad sacked local governor Ahmad Khaled Abdel Aziz for reportedly failing to suppress the unrest.
Security forces swept in on Monday in an attempt to regain control of the city, but tanks have so far remained outside.