Human rights activists say heavy shooting has been heard in a western suburb of the Syrian capital, Damascus, since the army cordoned off the area.
Security forces are also continuing their efforts to crush anti-government protests in the central city of Homs, the southern city of Deraa and the coastal town of Baniyas.
Foreign journalists have not been allowed to enter Syria, the BBC reports, so the reports are difficult to verify independently.
On Monday morning, reports from activist websites said that the suburb of Muadhamiya, to the west of Damascus, was cut off by troops.
Heavy gunfire could be heard and clouds of black smoke were hanging over the area. Electricity and telephone lines were cut off.
Security forces are also said to have moved into Homs, north of Damascus, where troops backed by tanks have been raiding houses and arresting people since Saturday night.
On Monday, a resident told the BBC that the security forces were dividing up the city to prevent people joining any mass protests.
"There is the sound of tank-fire and gunfire coming from the outskirts of the city," the resident said. "Last night, there was continuous gunfire for several hours. There is no electricity or mobile phone coverage."
Death toll of 800 estimated
The British-based Syrian Observatory for Human Rights said on Sunday that the areas of Bab Sebaa, Bab Amro and Tal al-Sour were under "total siege", and that hundreds had been arrested. The number of casualties was unknown, it added.
About 15 people were said to have been shot dead in Homs on Friday after taking part in demonstrations following Friday prayers.
The Syrian human rights group Sawasiah believes more than 800 people have been killed since pro-democracy protests began in March, including 220 killed in an army assault on Deraa.
The government disputes the civilian toll and says about 100 soldiers have been killed.
The unrest poses the most serious challenge to President Bashar al-Assad since he succeeded his father, Hafez, in 2000.