Story highlights The dead include two nurses, says rebel leader

Dar al-Shifa Hospital is a primary source of medical help in Syria's commercial hub

Its maternity ward had been attacked earlier in the year

At least 40 people were killed in an airstrike on a key hospital in the besieged city of Aleppo, a rebel leader there told CNN.

Ralib al-Omar, with the Yusif al-Asma rebel group, said the attack targeted the Dar al-Shifa Hospital and that the dead included two nurses.

The hospital is one of the main sources of medical help for people in Syria's commercial hub.

Video posted by activists showed a collapsed building adjacent to the hospital. The blast appeared to have hit the hospital's front lobby, which is often crowded with patients, civilians and rebels.

The hospital had been attacked this year when an artillery shell hit its maternity ward.