Four people were killed Saturday after a fire broke out at a construction site in the western city of Gimpo, rescue workers said.

The fire broke out at about 1:30 p.m. at the construction site of a residential building here, according to officials from the Gimpo fire station.

What exactly started the fire will be determined through a later investigation, but the officials believe exposed insulation may have caught sparks from a welder as workers at the construction site have said they were welding pipelines on the second basement floor of the building where the fire started.

All four deceased workers were found on the second basement floor. An initial diagnosis suggested all four suffocated after inhaling toxic gas.

"It was a simple fire, but we believe the casualties were so high because polyurethane foam creates dangerously toxic gas when burning," an official said.

Two others have been found and taken to a nearby hospital for treatment, and they are both said to be in critical condition.

Rescue workers are also searching the area for an additional worker believed to be missing as other workers at the accident said a total of seven had been working on the second basement floor when the fire broke out. (Yonhap)