More than 150 people have been killed since a Muslim holiday truce between rebels and the regime came into effect, a watchdog said on Saturday, as new clashes broke out across Syria.

The four-day conditional ceasefire between President Bashar al-Assad's regime and the rebel Free Syrian Army for the Muslim Eid al-Adha holiday started Friday but has failed to take hold amid sustained clashes, bombings and shelling.

On Saturday, clashes and attacks took place in the Damascus province, Aleppo to the north, Daraa in the south and the eastern city of Deir Ezzor, the Syrian Observatory for Human Rights said.

Five people were killed in a car bomb attack in Deir Ezzor, the Observatory said. State television blamed the attack on "terrorists" and said the bomb had gone off in front of a church, causing significant damage.

Regime shelling targeted several areas of Damascus province, where one person was killed by army sniper fire and another in the bombardment, it said. Two soldiers were also killed in clashes in the rebel-held city of Harasta near Damascus.

Observatory director Rami Abdel Rahman said violence east of Damascus also continued to rage despite the truce proposal.

"In the Eastern Ghuta area of Damascus province, the army has used tanks to shell several rebel-held towns, including Harasta, Zamalka and Douma," Abdel Rahman told AFP.

"The army's bid to take back rebel areas east of the capital has not ceased despite the truce," he added.

Two people, including a child, were also killed by gunfire in Daraa, while shelling of various areas of Aleppo wounded several people, the Observatory said.

An AFP journalist in Damascus said warplanes were flying intensively over the capital, but there were no reports of air strikes.

The Observatory said a warplane had also been seen over Aleppo, the country's commercial capital and the scene of fierce fighting since July.

At least 146 people were killed on Friday and another 13 were reported dead on Saturday, the Observatory reported.

According to the watchdog, more than 35,000 people have been killed in 19 months of conflict, which began as an anti-regime uprising but is now a civil war pitting mainly Sunni rebels against Assad's regime dominated by his minority Alawite sect, an offshoot of Shiite Islam.

The Britain-based Observatory relies on a countrywide network of activists and medics in civilian and military hospitals. It says its tolls take into account civilian, military, and rebel casualties.