The drill was conducted at the country's four nuclear plant complexes, said Korea Hydro and Nuclear Power Co (KHNP), which operates 23 nuclear reactors and supplies about 30 percent of the country's electricity needs.

"The two-day drill is under way through simulators to ensure the safety of our nuclear power plants under cyber attacks," said KHNP spokesman Kim Tae-Seok.

Last week a suspected hacker, using an account entitled "president of the anti-nuclear reactor group", published a variety of information on a Twitter account.

It included designs and manuals for two reactors as well as personal information on some 10,000 KHNP workers.

In a new message yesterday, the hacker threatened to release more information unless the government shuts down three reactors from December 25. The message warned residents in areas near the plants to stay away for the next few months.

State prosecutors launched an investigation but failed to find out who was responsible. Vice energy minister Lee Kwan-Sup confirmed today that the information leaked so far appeared to be from the plants at Gori and Wolsong southeast of Seoul.

The government has been handling this case with "extreme care", although no sensitive information has been leaked so far, he said. Lee said the leaks would not have any impact on the safety of the reactors.

The drill comes as South Korea is on guard against possible cyber attacks by North Korea after President Barack Obama said it was confirmed that the North carried out the hacking of Sony Pictures in the United States.

The FBI has cited "significant overlap" between the attack and other "malicious cyber-activity" with direct links to Pyongyang, including an attack on South Korean banks blamed on the North.

North Korea has denied involvement in the hacking of Sony Pictures, which prompted executives to halt the release of a film seen by Pyongyang as mocking its leader.

But the North threatened yesterday to hit back at the White House and other US targets if Washington sanctions it.

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