One worker has been confirmed dead and another is feared dead among the seven trapped after a boiler tower collapsed at a thermal power plant in the southeastern city of Ulsan.

Local fire authorities said Friday that one worker was pronounced dead at 4:53 a.m. after suffering cardiac arrest during rescue operations, marking the first fatality from the incident.

Rescuers made 12 attempts to reach the trapped worker, administering painkillers and providing thermal protection, but were ultimately unable to save him.

The victim was one of two workers whose locations had been confirmed, with the other also believed to be dead.

The 60-meter-high boiler tower at the plant, managed by the Ulsan branch of the state-run Korea East-West Power Company, collapsed around 2 p.m. Thursday, trapping nine workers.

Two have been rescued, while six remain trapped.