Syria has accused Washington of inciting the unrest in the Arab nation.
The US ambassador's visit to the flashpoint city of Hama proves that Washington has a hand in the country's troubles, said a Foreign Ministry official .
The report on Syria's state-run news agency says Ambassador Robert Ford travelled to Hama without permission.
The State Department said Mr Ford travelled independently to Hama to show solidarity with residents protesting against President Bashar Assad's regime.
Mr Ford took part in a government-organised excursion to northern Syria last month.