Syrian opposition activists reported on Saturday a return to heavy government bombardment in major cities, further undermining a truce intended to mark the Muslim Eid al-Adha religious holiday.

According to reports, more than 150 people were killed since the ceasfire began on Friday.  

The Syrian Observatory for Human Rights, a British-based opposition organization with a network of sources within Syria, reports that most were shot by sniper fire or in clashes. Forty-three soldiers were killed in ambushes and during clashes, it added, and state TV reported a powerful car bomb which killed five people in Damascus.

The organization also reported later Saturday that Syrian warplanes had bombed a building in a suburb of the capital Damascus, in the first airstrike since the internationally-mediated cease-fire began.

Eight people were reportedly killed and many others wounded in the airstrike in Arbeen. The area also has witnessed heavy clashes and intense shelling. An amateur video posted by activists online shows a pile of rubble said to be from the airstrike. A hand of what appeared to be a dead person could be seen in the debris. The videos appeared consistent with AP's reporting in the area.

Activists in the eastern city of Deir al-Zor, the suburbs of Damascus and in Aleppo, where rebels hold roughly half of Syria's most populous city, said that mortar bombs were being fired into residential areas on Saturday morning.

The bombardment came on the second day of a truce called by international peace envoy Lakhdar Brahimi, who had hoped to use it to build broader moves towards ending the 19-month-old conflict which has killed an estimated 32,000 people.

"The army began firing mortars at 7 A.M. I have counted 15 explosions in one hour and we already have two civilians killed," said Mohammed Doumany, an activist from the Damascus suburb of Douma, where pockets of rebels are based. "I can't see any difference from before the truce and now," he added.

The Syrian military has said it responded to attacks by insurgents on army positions on Friday, in line with its announcement on Thursday that it would cease military activity during the holiday but reserved the right to react to rebel actions.

A statement from the General Command of the Armed Forces detailed several cease-fire violations in which it said "terrorists" had fired on checkpoints and bombed a military police patrol in Aleppo.

Violence had initially appeared to wane in some areas on Friday but truce breaches by both sides swiftly marred Syrians' hopes of celebrating Eid al-Adha, the climax of the Haj pilgrimage to Mecca, in peace. Brahimi's cease-fire appeal had won widespread international support, including from Russia, China and Iran, President Assad's main foreign allies.

The war in Syria pits mainly Sunni Muslim rebels against Assad, from the minority Alawite sect which is distantly related to Shi'ite Islam. Brahimi has warned that the conflict could suck in Sunni and Shi'ite powers across the Middle East.

Brahimi's predecessor, former UN chief Kofi Annan, declared a cease-fire in Syria on April 12, but it soon became a dead letter, along with the rest of his six-point peace plan.