At least 70 people were killed in a string of barrel bomb attacks Saturday on the Syrian city of Aleppo, according to the London-based Syrian Observatory for Human Rights and the opposition-run Aleppo Media Center.
Human rights groups say the Syrian government has used barrel bombs -- oil drums filled with explosives and shrapnel -- in areas with civilian populations, killing thousands.
President Bashar al-Assad has steadfastly denied that his government has used barrel bombs.
CNN cannot independently confirm the reports by the observatory and Aleppo Media Center.
According to those opposition groups, the deadliest of Saturday's strikes killed 59 people -- many of them civilians -- in a busy market area in al-Bab, a northeastern city under the control of ISIS.
Another attack on a rebel-held quarter of Aleppo left 12 people dead, including three children and eight women, according to the Syrian observatory.
Activists posted video of a third attack that showed rescue workers and civilians in the northern neighborhood of Fardos scrambling to pull bloodied children out of pancaked buildings.
At least 20 people were killed in the attack and tens of others wounded, according to the Aleppo Media Center.
U.N. special envoy for Syria Staffan de Mistura condemned the reported attacks.
"The use of barrel bombs must stop," Mistura said in a statement. "All evidence shows that the overwhelming majority of the civilian victims in the Syrian conflict have been caused by the use of such indiscriminate aerial weapons."
Earlier this month, rights group Amnesty International alleged in a report that Syrian government forces had killed and wounded civilians by dropping barrel bombs on schools, hospitals, mosques and crowded markets in Aleppo.
Attacks using barrel bombs killed more than 3,000 civilians in Aleppo governorate last year, the report said, and have helped create a climate of fear.
Also Saturday, farther north in Aleppo's Hasakah province, ISIS launched an offensive against Syrian troops that left around 50 soldiers and 10 jihadis dead, according to the Syrian observatory.
Extremists detonated two booby-trapped cars at Syrian army positions. Control of the northeastern city is split between government forces and Kurdish militia.