Three South Korean workers have died after apparently inhaling toxic gas at a construction site for a new nuclear plant.

Choi Hee-ye of Korea Hydro and Nuclear Power Co said the victims were working at the construction site in the southeastern city of Ulsan when they fell unconscious and were taken to a hospital.

Choi said the company has yet to determine the cause of the accident, although the leaking of nitrogen was suspected.

Construction of the plant is scheduled to be completed next year.

The accident comes as the company is on high alert over a series of threats by hackers who claim they can disable the control systems of its plants.