Four workers were killed and 10 others were injured Wednesday, when a subway construction site near Seoul collapsed due to what is believed to be a gas explosion, police said.

The victims, who were trapped under debris 15 meters underground, were transported to nearby hospitals for treatment, according to authorities.

The accident, which occurred in the city of Namyangju, east of Seoul, around 7:30 a.m., took place when a total of 17 workers were on duty. One of the victims died after being thrown by the explosion. The three others were found dead under the collapsed construction site, firefighters said

A witness said there was a huge blast, but the police said they believe explosives were not being used at the time of the collapse.

The number of deaths could increase as some of the injured are in critical condition, the police said.

"We set up a situation room at the site of the accident," said an official at POSCO E&C, the construction unit of the world's fourth-largest steelmaker, who was in charge of the construction site. Whether the victims were from the company or from its subcontractor has not been clarified yet. 

Police and firefighters said they are investigating the exact cause of the accident. (Yonhap)