Syrian warplanes flattened a building next to a hospital in Aleppo, killing at least 15 people and damaging one of the last remaining sources of medical help for civilians in the northern city, activists said Thursday.

Once a private clinic owned by a businessman loyal to President Bashar Assad, the Dar al-Shifa hospital became a field hospital run by volunteer doctors, nurses and aides united by their opposition to the regime and the need to give medical care to both civilians and rebels.

The facility has taken at least six direct hits in recent months, mostly affecting the upper stories.

On Wednesday night, warplanes bombed a building adjacent to the hospital, turning it into a pile of rubble and spraying shrapnel and debris into Dar al-Shifa itself, activists said.

Rami Abdul-Rahman, chief of the Britain-based Syrian Observatory for Human Rights, said at least 11 fighters were killed in the airstrike, in addition to a doctor, a young girl and two other children who were on the street.

Another activist group, the Local Coordination Committees, confirmed the bombing and identified the doctor as Mohammad Qassem Agha. The group said 40 people died in airstrikes in Aleppo on Wednesday, but did not say how many died in the hospital strike.

Videos posted online by activists showed the flattened building and substantial destruction in front of the hospital. Residents and armed rebels, and in one instance, a doctor wearing green scrubs are seen picking through the rubble and overturned gurneys at the entrance of the hospital.

In one video, a man is seen calling for survivors under the rubble while a man is heard crying for help underneath a huge slab of concrete.

The seven-storey hospital stands only 400-500 metres from the front line in a neighbourhood that is heavily shelled on daily basis.

Aleppo, Syria's business and commercial hub, has been locked in a deadly stalemate for months between regime forces and rebels seeking to topple Assad.

Syria's conflict erupted in March 2011 with an uprising against Assad's regime, inspired by other Arab Spring revolts.

The crisis has since morphed into a civil war, with scores of rebel groups across the country fighting government troops. Nearly 40,000 people have been killed in the 20 months of unrest, according to activists.