A string of rebel strongholds, including in Daraa and Homs provinces and the commercial capital Aleppo, were rocked by shelling and clashes from dawn on Monday that left at least 23 people dead, a watchdog said.
Twenty of the deaths occurred when the army launched an intensive pre-dawn assault on the town of Karak al-Sharqi in the southern province of Daraa, the Syrian Observatory for Human Rights said.
Two civilians meanwhile were killed when troops rained shells down on pockets of rebel resistance in Aleppo, Syria's commercial capital, while another civilian died when the town of Kfar Zeita, in the central province of Hama, was bombed, the Britain-based watchdog said.
Among those killed in Karak al-Sharqi were at least five rebel fighters, said the Observatory, which added that some of the deaths occurred when troops blasted cars ferrying wounded people to field hospitals and clinics for medical attention.
"Karak al-Sharqi has suffered repeated military assaults, heavy shelling and attempts to storm it over the past three days," said the Observatory, which collates its information from a network of activists and medics on the ground.
It added that the town was facing "a crippling blockade and terrible medical and humanitarian conditions."
In the central city of Homs, the army bombarded the rebel stronghold of Khaldiyeh from the early morning using warplanes, tanks, shells and mortars in a concerted multi-pronged attack, the Observatory said.
"There is heavy shelling as well as fierce clashes around the district, but the rebels are resisting the army onslaught," Observatory director Rami Abdel Rahman told AFP.
Elsewhere in Homs province, the town of Houla came under heavy bombardment by government forces, which left an unknown number of wounded, he said.
In March, at least 108 people, including 49 children and 34 women, were killed during a two-day army offensive that began with heavy shelling.
The northern city of Aleppo, which since mid-July has been the focal point of the revolt against the regime of President Bashar al-Assad, was rattled from early morning by shellings which hit a string of rebel-held neighbourhoods as well as by firefights between rebels and troops, the watchdog said.
Among the neighbourhoods pounded were Tariq al-Bab, Hanano and Sakhur in the east, and the southwest districts of Bustan al-Qasr, Ansari, Fardus, Kalasseh and Sukari.
Fighting raged in the central district of Midan, Sakhur in the east and Salaheddin and Saif al-Dawla in the southwest, the Observatory said.
The violence comes a day after 15 people, including five children, were killed in shelling across Aleppo.
On Sunday night, a car bomb struck the Damascus police headquarters, killing a policeman, according to the official news agency SANA.
Abdel Rahman of the Observatory confirmed the toll.
"Not more than one or two people were killed in the blast," he said.
The shelling and clashes on Monday followed a bloody day in which 120 people -- 54 civilians, 27 rebels and 39 government troops -- were killed nationwide, according to the Observatory.
Not included in the toll were 28 bodies, including three rebels, found in the Damascus town of Hameh after several days of military operations that ended in the army taking control of the area.
More than 31,000 people have been killed in violence in Syria since the outbreak in March last year of an anti-regime revolt, which began as peaceful protests for reform but morphed into an armed insurgency when demonstrations were brutally crushed.
Most rebels, like the population, are Sunni in a country dominated by a minority Alawite regime. Alawites are an offshoot of Shiite Islam.