The death toll from the collapse of the roof of a five-story cement factory in Bangladesh rose to at least seven Friday, a government official said.

M. Jahangir Alam, the local chief government administrator in Mongla in Bagerhat district where the accident happened Thursday, said rescue work was underway but there was no estimate for the number of people feared trapped.

About 150 people were working when the collapse happened at the seaside factory 135 kilometers (85 miles) southwest of Dhaka.

The cause of the collapse was under investigation.

Survivors said 50 to 60 people were working on the roof while others were on the ground floor when the collapse occurred. It was not clear how many were directly affected by the collapse.

The factory is owned by an army welfare organization, the United News of Bangladesh news agency reported.

Building structural safety is a problem in Bangladesh, with engineering surveys showing that many factories violate building codes by using substandard materials or adding extra floors that the building foundations can't support.

In April 2013, a building housing numerous garment factories collapsed in Dhaka, killing more than 1,100 people.