SEOUL - Authorities in South Korea fear five people remain trapped on Thursday after the collapse of a large structure at a power station in the city of Ulsan, a fire official said, although four others were pulled from the rubble.

Footage from the scene showed a massive steel structure mangled and toppled over in the southeastern city surrounded by similar structures.

A fire official, Kim Jung-shik, told reporters that workers were in the process of taking down parts of the structure, a decommissioned heating facility, to prepare it for demolition in about 10 days when the accident occurred.

Two people were quickly rescued and then two others were pulled from the rubble, Kim said. A rescue operation was ongoing, he said.