At least 72 people have been killed in Syria's northern Aleppo province by barrel bombs dropped from government helicopters, activists say.
The UK-based Syrian Observatory for Human Rights says 60 people died in the strike on al-Bab - a town currently held by Islamic State (IS) militants.
It says 12 were killed in a rebel-held quarter of the city of Aleppo.
Syria's government has repeatedly denied using barrel bombs - large containers filled with explosives.
The Syrian Observatory for Human Rights (SOHR), which gathers information through a network of activists in Syria, called it one of the worst massacres perpetrated by the government so far this year.
Meanwhile, Islamic State is reported to have blown up Tadmur prison near the ancient city of Palmyra - which fell to the militants earlier this month.
The prison was for decades a symbol of state oppression in Syria. It had held thousands of political prisoners, who faced years of torture and disease in its cells. Many were executed.
The SOHR says at least 60 people were killed in the attack on al-Bab's busy market on Saturday morning after government helicopters dropped two barrel bombs in quick succession.
Many of the victims were blown to pieces or burnt beyond recognition by the blasts, which devastated the market, shops and vehicles, reports the BBC's Jim Muir from neighbouring Lebanon.
Most of those killed or injured were men, because women do not appear much in public in areas controlled by IS, as al-Bab has been for some time, our correspondent adds.
In Aleppo, a man told the BBC he ran to the site of the barrel-bomb attack to find a vehicle on fire with around eight civilians dead inside it.
Activists report barrel bombs being dropped from government helicopters every day in different parts of the country.
They consist of steel drums packed with explosives and shrapnel - and sometimes with chlorine also added, according to many reports.
They are highly inaccurate, and often cause massive damage and indiscriminate casualties in built-up areas.
The Syrian government has made no public comment on the reports, which have not been independently verified.
In February, Human Rights Watch group accused Damascus of dropping barrel bombs on hundreds of sites in 2014, violating a UN Security resolution.
In an interview with the BBC also in February, President Bashar al-Assad denied his forces used barrel bombs. Dismissing the allegation as a "childish story", he said: "There are no barrel bombs. We don't have barrels."
Aleppo has been divided between government- and rebel-held areas since fighting erupted in the former commercial hub in 2012.
More than 220,000 people have been killed across Syria since the start of the uprising against President Bashar in 2011, according to UN estimates.