At least 20 people were killed on Tuesday in a regime barrel bomb attack on a mini-bus stand in a rebel-held part of Syria’s Aleppo city, a monitor quoted by AFP said.

“Helicopters committed a massacre, dropping a barrel bomb on a mini-bus station in the Fardous district of Aleppo, killing 20 civilians, among them children, and injuring 30 more,” the Syrian Observatory for Human Rights group said.

The Britain-based monitor said the toll was expected to rise because of the number of serious injuries among the wounded.

The group said most of bodies of those killed “were charred”.

Video footage distributed by activists and posted on YouTube showed rubble and twisted scraps of metal strewn across the cratered pavement where the station was, underneath a bridge.

Firefighters and civilians worked with hoses to tackle fierce blazes in several vehicles that produced thick clouds of smoke, according to AFP. A least one car had been flipped on its side.

A second video showed several men hoisting a male victim missing a large section of the back of his torso onto an orange stretcher and into an ambulance.

It was unclear whether he was still alive.

Once Syria’s economic powerhouse, Aleppo has been divided between government control in the city’s west and rebel control in the east since shortly after fighting there began in mid-2012.

For months, the regime has waged an unrelenting aerial campaign against the opposition-held part of the city, making particular use of crude so-called barrel bombs.

Rights groups have criticized the weapons as indiscriminate and said they are more likely to cause civilian casualties.

Last week, in fact, Amnesty International accused Syrian government forces of committing “crimes against humanity” by indiscriminately bombing Aleppo.

Despite documentation of the government’s use of the weapons by rights groups and activists on the ground, Syrian President Bashar Al-Assad has repeatedly denied that his forces use barrel bombs.

In a recent interview, Assad asked “what are barrel bombs?” when asked about his army’s use of the weapon.