SEOUL—A worker was killed and six others remained trapped after a power plant structure collapsed on South Korea's east coast, the Yonhap news agency reported Friday, citing firefighters.

The boiler tower collapsed around 2:00 pm (0500 GMT) Thursday at Korea East-West Power's Ulsan Power headquarters in the southeastern city of Ulsan.

"While under rescue operation, one worker was pronounced dead due to cardiac arrest," Yonhap reported, citing local firefighting authorities.

The National Fire Agency earlier said two people had been rescued, while at least seven others were trapped and rescue operations were underway.

South Korean Prime Minister Kim Min-seok ordered all ministries to "mobilise all available personnel and equipment, placing top priority on rescuing those trapped."

Residents were also being evacuated from the area, the prime minister's office added.

The accident reportedly occurred during preparations to demolish the thermal power plant, which