The army bombarded opposition strongholds in the city of Homs and the towns of Qusair, Talbiseh and Rastan. Free Syrian Army rebels had been intensifying attacks on army patrols, roadblocks and missile batteries in the area, the Syrian Network for Human Rights and other opposition campaigners said.
Abu Qassem, an activist in Rastan, 25 km (15 miles) north of Homs, said at least 500 rockets and shells had fallen on the town since Saturday, and army helicopters were firing machineguns into the area.
"The Free Syrian Army is far outgunned, but it is responding by mounting guerrilla attacks while trying to avoid direct exchange of fire," he said.
He said among the rebels' targets had been an army missile battery in the area of Ghanto near Rastan. Most members of the missile squadron defected and the battery fell under rebel control.
Rastan was once a reservoir of Sunni Muslim recruits for the military, whose senior ranks are dominated by members of Assad's minority Alawite sect, an offshoot of Shi'ite Islam.
After Syria's revolt broke out in March last year and pro-democracy demonstrators in Rastan were killed, Sunni officers from the town began defecting.
Talbiseh to the south came under shelling and heavy mortar fire from loyalist troops after some soldiers from surrounding roadblocks defected on Saturday and drove two armoured personnel carriers into the town, according to opposition sources there.
"Five people have been killed, including a woman and her one-year-old daughter. They were among the few civilians who had not fled Talbiseh," activist Abu Mohammad said by satellite phone.
In Homs, the army shelling was concentrated on the neighbourhood of al Khalidiya, inhabited mostly by Sunni tribal families from the desert to the southeast of Homs, activists said.