Damascus: Ten civilians killed by regime mortar fire and 12 troops killed in clashes with rebels were among 24 dead in Syria on Tuesday just hours after a new UN call for all sides to respect a troubled truce.

UN chief Ban Ki-moon said it was vital that government and opposition alike cooperate fully with a hard-won UN observer force as it fans out in its mission to shore up the ceasefire agreement that took effect on April 12.

But despite his appeal, human rights monitors reported no let-up in the violence, a day after a series of blasts condemned by Ban as "terrorist bomb attacks" killed more than 20 people.

Nine members of a single family were among the 10 killed in the regime bombardment of a village in Idlib province in the northwest, the Britain-based Syrian Observatory for Human Rights said.

A mortar round slammed into their home in Mashmashan village near the town of Jisr al-Shughur, the watchdog said, adding that four women and two children were among the dead.

Outside Damascus, an 80-year-old man was killed in the town of Qatna by gunfire from a minibus, the Observatory said. It cited residents as saying the vehicle belonged to the security forces.

In Deir Ezzor province in the northeast, 12 soldiers were killed in clashes with rebel fighters, the watchdog added.

One civilian was also killed during the fighting in the Busayra area of the province, in which troops responded with missile and heavy machinegun fire, it said.

Ban called on all sides to work with the UN observer mission as it expands from the current advance party of 30 to a promised full complement of 300 in the coming days and weeks.