Beirut: Syrian warplanes bombed the rebel-held town of Douma near Damascus and parts of Aleppo in the north on Saturday, killing 23 people, with the death toll likely to rise, the Syrian Observatory for Human Rights said.
Mediators have struggled to get combatants in Syria's five-year-old war to honour a February 27 cessation of hostilities deal to enable peace talks in Geneva to proceed.
Each side accuses the other of violating the truce.
Fighting has escalated around Aleppo, Idlib, Latakia, Damascus and other areas over the past week and the main opposition group walked out of Geneva peace talks this week in protest at government attacks.
The Geneva talks aim to end a war that has killed more than 250,000 people, created the world's worst refugee crisis, allowed for the rise of the IS extremist group and drawn in regional and major powers.
Russia's intervention in the conflict beginning late last year has swayed the war in President Bashar Al Assad's favour.
The Britain-based Observatory, which monitors the Syrian war through a network of contacts, said the death toll in Douma, northeast of the capital, was expected to rise from 13 because more than 22 others were injured, some critically.
In a government-controlled camp near Douma, shelling killed a woman and child, and injured others, the Observatory said.
There was also fighting near Bala in the southeast of Damascus between rebel groups and government forces with deaths occurring on both sides.
In Aleppo, at least ten people were killed, including a child, by bombs dropped from planes in an insurgent-controlled eastern neighbourhood of what was Syria's commercial hub before the civil war began in 2011.
This is the second day of heavy bombardment on Aleppo.
Nineteen people were killed on Friday in similar air attacks.
In a government-held area of northwest Aleppo, Syrian state television said six people were injured in rebel shelling.
On Friday a Syrian warplane crashed southeast of Damascus.
The Syrian military said it crashed because of a technical fault, but Islamic State said it shot the plane down and had taken its pilot captive.
In a statement on Saturday the extremist group said this was the third Syrian warplane it had shot down in two weeks, in addition to a Russian drone.
On Friday, the UN special envoy for Syria vowed to take the talks into next week despite the opposition suspending their involvement.