The crackdown on opposition supporters in Syria continues.
Residents said troops backed by tanks raided houses in two districts of Hama overnight.
A local activist told Reuters that gunfire could be heard.
Another resident said pick-up trucks equipped with machine-guns and buses full of troops had grouped at a northern entrance to the city.
Syrian authorities said the army had withdrawn from Hama this month after a 10-day assault to put down pro-democracy protests.
Hama, some 200 kms north of the capital Damascus, was the scene of a 1982 massacre by the military.
The fresh violence comes after activists said security forces had shot dead four demonstrators in southern Deraa province on Aug. 30 as people left mosques after prayers to mark the end of Ramadan.
In Washington, the Obama administration froze the U.S. assets of Foreign Minister Walid al-Moualem and two other Syrian officials in reaction to President Bashar al-Assad's increasingly bloody crackdown.
Amnesty International said that deaths in Syrian prisons and police detention have rocketed up in recent months.
The London-based human rights group said it had details of at least 88 people believed to have died in detention between April and mid-August.
compiled from agency reports