Syrian troops stormed the central province of Hama on Tuesday, with a heavy barrage of shelling, activists in the area told dpa.
"The tanks have seized all entrances to the city and all communication networks were cut," said Abu Abdullah, a member of the Local Coordination Committees (LCC), which organizes protests on the ground.
"The people are trying to flee the targeted areas but the army has set up checkpoints to prevent anyone from leaving the city," he added.
In the northern province of Idlib, security forces used fire to disperse around 10,000 mourners who took to the streets Tuesday for a funeral procession of a protester killed the day before, the Syrian Observatory for Human Rights said.
The military operation came a day after Syria rejected an Arab League proposal to end the country's unrest, saying that the call for President Bashar al-Assad to leave office after a unity government is formed amounted to "blatant interference" in national affairs.
The Arab League on Sunday called on al-Assad to step down and hand over power to his vice president under the terms of a transition plan similar to one which paved the way for the departure of Yemen's President Ali Abdullah Saleh from power.
The Syrian regime have been blaming the uprising against al-Assad, which began in March, on terrorists and armed gangs financed by foreign countries. According to United Nations estimates, more than 5,400 people have been killed.