At least 42 children have been killed in government air strikes and shelling across Syria in the last 36 hours, the Syrian Observatory for Human Rights monitoring group said Sunday.

The Britain-based Observatory said 25 children had been killed between midnight on Saturday and Sunday afternoon, with 17 more killed between Friday and Saturday night.

The deaths came in regime shelling and air strikes across the country, though most took place in the northern province of Aleppo and northwestern Idlib, Observatory director Rami Abdul Rahman said.

Many of the deaths came in raids involving the use of explosive-packed barrel-bombs, a weapon that has been criticised by rights groups as indiscriminate.

Among the dead on Sunday were at least five children killed along with five adults in a barrel bomb attack on the town of Hobait in Idlib province, said the monitor.

In northern Aleppo province, another five children and three adults were killed in an air raid in the west of the province, it added.

In the capital Damascus, meanwhile, regime planes continued to pound the eastern rebel-held district of Jubar, where the government began a fierce offensive earlier this week to wrest back control.

The Observatory said at least 15 air raids hit the district on Sunday, but there were no immediate details about casualties.

Jubar has been in insurgent hands for a year, and is considered strategic because it provides a gateway to the centre of the capital and opens onto the key rebel stronghold of Eastern Ghouta.

In mid-August, the army took Mleiha some 10 kilometres (6 miles) southeast of Damascus, and capturing Jubar would allow a two-pronged advance on Eastern Ghouta.

Rebels arrayed around the capital regularly fire mortar and rockets into Damascus.

In northern Syria, meanwhile, the Observatory said it had documented the executions of 31 rebel fighters and civilians by Islamic State jihadists in the last 10 days of August.

Among those killed were rebel fighters, militants from the rival jihadist group Al-Nusra Front and civilians, including a university professor executed on allegations including membership in Syria's ruling Baath party, said the monitor.

The figure does not include hundreds of regime forces and members of the Sunni Shaitat tribe who were executed after battles with IS jihadists in Raqa and Deir Ezzor provinces.

It also does not include rebels killed in clashes with IS, which has been battling rival opposition fighters since early this year.

More than 191,000 people have been killed in Syria since the conflict began there in March 2011, the UN says.