Seoul: Computer systems at South Korea's nuclear plant operator have been hacked, the company said on Monday, sharply raising concerns about safeguards around nuclear facilities in a country that remains technically at war with North Korea.

The Korea Hydro and Nuclear Power Co Ltd (KHNP) and the government said only "non-critical" data was stolen by the hackers, and that there was no risk to nuclear installations, including the country's 23 atomic reactors.

On alert ... South Korean employees conduct a simulated drill to ensure the safety of nuclear power plants under cyber attacks, at a training center of the Wolseong Nuclear Power Plant in the southeastern city of Gyeongju on Monday. Credit:AFP

But the hacking was reported as the United States accused North Korea of a devastating cyberattack on Sony Pictures.

Experts voiced alarm that the controls of the nuclear reactors could be at risk.