At least 21 people died when a boiler exploded in a four-story Bangladeshi factory near Dhaka early Saturday, authorities said.

A fire broke out after the explosion, which happened while employees were inside the facility in Gazipur district, local police chief Harunur Rashid said.

Fifty people are hospitalized with injuries, and the death toll is expected to go up as rescue efforts continue, according to the police chief.

Hundreds of firefighters are at the scene attempting to extinguish the fire. The cause of the explosion is unknown.

(CNN)