Syrian forces pounded Aleppo and Deir Ezzor provinces as at least 35 people were killed on Sunday across the country, among them 17 civilians, a watchdog reported.
The regime forces, under the cover of heavy shelling, attempted to storm the rebel strongholds of Qusayr and Rastan in the central province of Homs, triggering fierce clashes, said the Syrian Observatory for Human Rights.
"Fierce clashes took place before dawn between regime forces and rebels in the vicinity of Qusayr, accompanied by heavy shelling on Qusayr and neighbouring villages as the regime tried to regain control," Observatory director Rami Abdel Rahman told AFP by phone.
In Rastan, "the army carried out shelling in an attempt to storm the city and clashed with fighters from the rebel combat battalions, forcing the government troops to retreat," he said.
Both towns are strongholds of the rebel Free Syrian Army.
Local Coordinating Committees, made up of anti-regime activists on the ground, said explosions rocked Qusayr, located near the border with Lebanon, amid random shelling and shooting.
The army "renewed shelling on residential areas using artillery and tanks," the LCC reported.
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Regime forces also shelled villages in the eastern province of Deir Ezzor, killing five people, including three children, as well as Aazaz town in the northern province of Aleppo and neighbourhoods of the central city of Homs, according to the Observatory.
The highest casualties were reported in central province of Hama, where six people were killed in gunfire in the village of Sahel al-Ghab, it added.
In addition, nine regime troops were killed in clashes with rebels in the provinces of Daraa, Deir Ezzor and Homs, the watchdog said.
The violence followed a bloody day in which 77 people were killed on Saturday, among them 39 civilians, 25 soldiers and 13 rebels.
More than 17,000 people, including 11,815 civilians, have been killed in Syria since the outbreak of the revolt in March 2011, the Observatory reported Saturday.
The total also includes 4,316 members of the army and security services and 881 deserters, Abdel Rahman told AFP.
Rebels who are not army defectors are included in the civilian toll.
Since the beginning of an attempted ceasefire on April 12, a key point in the peace plan of UN-Arab League envoy Kofi Annan, 5,898 people have been killed, he said.
The tolls are impossible to verify, and the United Nations has stopped keeping a tally.
On Saturday, Annan acknowledged the failure so far of his mission to bring peace to Syria, adding that there was no guarantee that it would succeed.