At least 33 people, including 10 children, were killed in government barrel bomb attacks in northern Syria on Wednesday, a monitoring group said.
The Syrian Observatory for Human Rights said the deaths came in three incidents, in Aleppo province in the north of the country and Idlib province in the northwest.
In Tal Rifaat in Aleppo, 16 people were killed, including eight children, when government helicopters dropped at least four barrel bombs, the Britain-based monitor said.
In the rebel-held neighbourhood of Jubb al-Qubbeh in east Aleppo city, nine civilians were killed, among them two children, when a barrel bomb struck the residential area.
And in Idlib province, eight members of one family were killed in a barrel bomb attack in the town of Kafr Sijna.
The Observatory, which relies on a network of sources on the ground, said the tolls in the incidents were expected to rise because of the number of people seriously wounded.
Regime barrel bombs -- crude weapons made of containers packed with explosives -- have often struck schools, hospitals and markets in Syria.
Rights groups criticise them as indiscriminate, saying they kill a disproportionate number of civilians.
Syrian President Bashar al-Assad has denied that his forces use the weapons, but evidence collected by activists and rights groups includes footage of the barrels being pushed from army helicopters.
Human Rights Watch has also said there is "strong evidence" the regime has dropped barrel bombs containing toxic chemicals on northern Syria.
On Wednesday, the New York-based group said it had led an investigation into three attacks in Idlib province, which killed two people and affected 127 others, and that deadly chlorine was likely used in some, if not all, of them
"The Syrian government has used barrel bombs with toxic chemicals for more than a year while the (UN) Security Council has failed to act," said Philippe Bolopion, HRW's UN and crisis advocacy director.
Meanwhile, the extremist Islamic State group continued its assault on the northeast Syrian city of Hasakeh, detonating at least five car bombs Wednesday as it advanced towards the city.
"IS blew up five booby-trapped vehicles in various locations south of Hasakeh, and there are fierce clashes now," said Observatory head Rami Abdel Rahman.
Syria's state TV also reported the five car bombs, but said they had all struck a prison still under construction.
Abdel Rahman said IS had sent reinforcements of at least 400 fighters from the eastern province of Deir Ezzor to the Hasakeh assault, which began on May 30.
More than 220,000 people have been killed in Syria since the conflict began in March 2011 with anti-government demonstrations that were met with a regime crackdown.