Syrian government helicopters dropped a barrel bomb Tuesday in a neighborhood in the northern city of Aleppo, hitting a busy bus depot and killing at least 15 people, activists said.

The attack on the Jisr al-Haj neighborhood killed at least 15 people, said the Local Coordination Committees, a monitoring group. The Britain-based Syrian Observatory for Human Rights said at least 20 people were killed and 30 were wounded, including women and children.

Both groups are based outside Syria but rely on a network of activists on the ground. The Aleppo Media Center, based in the city, said at least 35 people were killed. Differing casualty figures are not uncommon.

The Aleppo Media Center posted pictures online purportedly from the bus depot, showing small fires as medics carried away burned bodies on stretchers, a stream of blood under their feet.

Aleppo, once Syria's largest city, is a major battleground in the country's civil war, carved into rebel- and government-held neighborhoods. Insurgents mostly control the countryside.

In a report this month, Amnesty International sharply condemned the government's reliance on barrel bombs, saying they killed over 3,000 civilians in Aleppo last year.

Meanwhile Tuesday, the international aid group Handicap International warned that Syria is so awash with weapons and bombs that the lives of 5.1 million people there, including 2 million children, are at constant risk. It called on all parties in the conflict to abide by international humanitarian law and immediately end the use of explosives in highly populated areas.

Syria's conflict, now in its fifth year, has killed more than 220,000 people. At least 1 million have been wounded.

Underlining the danger civilians face, two motorcycles rigged with explosives blew up in the central city of Homs, killing at least four people and wounding 28, Syrian state media said.

The Islamic State group claimed responsibility for the attacks, which it said were neighborhoods that are mostly inhabited by members of President Bashar Assad's minority Alawite sect.