Preliminary investigations suggest that fuel gas exploded, causing part of the two-floor apartment building to collapse, said a spokesman with the municipal government's information office.

The accident occurred at 8:27 am at the 5th Entrance of the No 3 apartment building in the 12th neighborhood of Heping East Street, Chaoyang District.

It's unknown how many people were inside the building when the explosion occurred, but according to fire fighters, the collapsed part of the structure contained six apartments.

Rescuers also evacuated nearby buildings and a school. Further investigation was underway, said the spokesman.