At least seven people were trapped after a power plant structure collapsed on South Korea's east coast, the National Fire Agency said on Thursday.

The collapse occurred around 2pm at Korea East-West Power's Ulsan Power headquarters in the southeastern city of Ulsan, authorities said, after a large structure collapsed.

The National Fire Agency said that two people have been rescued, while at least seven others are believed to be trapped and remain unaccounted for.

Search 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 has been out of operation. (AFP)