A nuclear reactor on South Korea's southeast coast was shut down Thursday afternoon, but the cause was not immediately known, plant operators said.

No immediate threats of a radiation leak were detected, according to the Korea Hydro and Nuclear Power Co.

The Gori Nuclear Power Plant, about 450 kilometers southeast of Seoul, halted operation at around 4:34 p.m., only hours after it resumed operation following a two-month maintenance that ended Wednesday.

An investigation was underway to identify the cause for the shutdown, officials from the state-run power company said.

"The plant halt has nothing do to with the nuclear reactor and turbine system, but it is presumed to be caused by a glitch in main transformer," an official at the company said on the condition of anonymity.

The 950,000-kilowatt reactor began its commercial operation in April, 1986. The Gori Nuclear Power Plant also houses five other reactors. (Yonhap News)