South Korean troops staged a rare exercise Wednesday to guard nuclear power plants against a possible attack by North Korean agents, the defence ministry said.

The drill involving hundreds of soldiers, anti-terror police, firefighters and government officials took place at the Gori complex on the southeast coast, the ministry said.

The plant has four reactors, including the first one built in 1978, and four more are under construction.

The drill -- inspected by Han Min-Koo, chairman of the Joint Chiefs of Staff -- simulated the infiltration of North Korean agents to bomb a nuclear power plant, a ministry spokesman told AFP.

"Our nation will face a very dangerous situation in case of emergencies of any kind at nuclear power plants," Han said in a statement, calling for "perfect disaster preparedness".

The nuclear crisis in neighbouring Japan has already prompted South Korea to run safety checks on its own plants.

South Korea operates 20 nuclear plants, which generate some 35 percent of its electricity needs, and plans to build 12 more over the next 14 years.

Its oldest reactor at Gori was coincidentally turned off Tuesday due to an electrical malfunction amid growing controversy over the extension of its life.

There were no safety risks to the 587,000-kilowatt reactor and no radiation leaks, state-run Korea Hydro and Nuclear Power said, adding it was repairing an electrical component.

The reactor was temporarily turned off in 2007 at the end of its 30-year operation life cycle. But operators received government approval last year to stay online for another 10 years after updates to safety systems.

Environmentalists have demanded its closure.

The nation has vowed to stick to its atomic power development programme despite heightened world concern following the Japanese crisis.

"South Korea's reliance on energy intensive industries make it effectively impossible to give up nuclear power," Knowledge Economy Minister Choi Joong-Kyung told a business forum Wednesday.