"The ceasefire is practically over. Damascus has been under brutal air raids since day one and hundreds of people have been arrested," said veteran opposition campaigner Fawaz Tello.
"Assad has been trying to use the truce to seize back control of areas of Damascus," said Tello, who is well connected with rebels.
Speaking from Berlin, Tello said Sunni districts in the city of Homs, 140 km north of Damascus, and surrounding countryside came under Syrian army shelling on Sunday.
Both sides in the 19-month-old conflict have violated the ceasefire intended to mark the Muslim religious holiday of Eid al-Adha. The truce, brokered by international peace envoy Lakhdar Brahimi, was supposed to come into effect on Friday, the first day of the four-day holiday.
Syrian authorities blame "armed terrorists" for breaking the truce and the opposition says a ceasefire is impossible while Assad continues to move his tanks and use heavy artillery and jets against populated areas.
Brahimi hopes to end the conflict that has killed at least 32,000 people and worsened instability in the Middle East. It began when a popular revolt broke out in March last year against four decades of authoritarian rule by Assad and his late father, President Hafez Assad.
The ceasefire appeal had won widespread international support, including from Russia, China and Iran, President Assad's main foreign allies.
But the truce seems destined to share the fate of failed peace efforts that have preceded it, with dozens of people continuing to be killed daily and international and regional powers at odds while they back different sides.
A sectarian divide between Assad's minority Alawite sect and Syria's majority Sunnis is also growing, fueling religious fervor in the region and driving more foreign jihadists into the country.
In the capital Damascus, activists and residents reported large explosions and plumes of smoke rising over the city as Syrian airforce jets bombed the suburbs of Zamalka, Irbin, Harasta and Zamalka.
A statement by the Harasta Media Office , an opposition activists' group, said aerial and ground bombardments had killed at least 45 people in the district since Friday.
Electricity, water and communications had been cut and dozens of wounded at the Harasta National Hospital had been moved as the bombardment closed in, the statement said.
Activists also reported fighting in the suburb of Douma to the northeast, where Free Syrian Army fighters have been attacking roadblocks manned by forces loyal to the government.
Assad is a member of the minority Alawite sect, which is distantly related to Shiite Islam. It has dominated majority-Sunni Syria since the 1960s, when Alawite officers assumed control of a military junta that had taken power in a coup.
Warplanes also hit towns and villages in the eastern province of Deir al-Zor, the northern provinces of Idlib and Aleppo, where rebels have been trying to push their advantage in rural areas by cutting off supply lines to the major cities, none of which has fallen completely under opposition control.Clashes with Kurds
Fighting was reported in the city of Aleppo, Syria's industrial and commercial hub. Rebels attacked several road blocks manned by Assad's loyalists and a 20-year-old girl was killed in army bombardment on Suleiman al-Halabi neighborhood, opposition activists said.
Rebel attempts to portray themselves as a united alternative to Assad suffered a setback when clashes broke out on Saturday in Ashrafieh, a Kurdish district of Aleppo that had up to now stayed out of the fighting. Armed clashes broke out between opposition fighters and members of the Syrian branch of the Kurdish Workers Party, or PKK.
Mouhaimen al-Rumaid, coordinator for the opposition Syrian Rebel Front, said the fighting erupted when PKK fighters helped Assad's forces defend a security compound in Ashrafieh that came under rebel attack.
Rumaid said scores of people were killed and rebels seized dozens of PKK members.
"The Ashrafieh incident has to be contained because it could extend to other areas in the northeast where the PKK is well organized," he said.
AMMAN (Reuters) -- Syrian jets bombarded Sunni Muslim regions in Damascus and across the country on Sunday, activists said, as President Bashar Assad kept up air strikes against rebels despite a UN-brokered truce that now appears to be in tatters.