A nuclear reactor at South Korea's Hanwool Nuclear Power Plant stopped operating Friday due to what appears to be a technical problem, plant operators said.

An investigation was underway to find the exact cause of the problem, but the state-run Korea Hydro and Nuclear Power Co. said there were no immediate safety threats.

The reactor at the power plant, previously called the Uljin Nuclear Power Plant, halted operations at 3:36 p.m.

The shutdown of the reactor with a generation capacity of 1 million kilowatts is expected to further aggravate the country's power supply.

Seoul was forced to shut down two nuclear reactors in late May after it was discovered that substandard control cables were used in the reactors under fake test certificates.

South Korea has 23 nuclear reactors that are operational, but nine of them are currently turned off for maintenance or inspection.

Since late May, the country has been struggling daily to keep its power reserves above what is considered a safe level of 4 million kilowatts. (Yonhap News)