South Korea's nuclear power plant operator launched a two-day drill Monday to test its ability to thwart a cyber attack, after a series of online information leaks by a suspected hacker.
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.
"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 published a variety of information on a Twitter account entitled "president of the anti-nuclear reactor group".
It included designs and manuals for two reactors, as well as personal information on some 10,000 KHNP workers.
In a new message on Sunday, the hacker threatened to release more information unless the government shuts down three reactors from December 25 and warned residents living near the plants to stay away for the next few months.
State prosecutors have launched an investigation but so far failed to find out who was responsible.
Vice energy minister Lee Kwan-Sup said the leaks appeared to be from plants at Gori and Wolsong, southeast of Seoul, but said they and would not affect the safety of the reactors.
KHNP insisted the suspected hacker could not have gained access to the plants' innermost networks, which were completely separated and isolated from unauthorised access.
"We believe the information must have been leaked before the networks were ... isolated last year," KHNP official Lee Jong-Ho told Yonhap news agency.
"The hacking of nuclear power plants is a serious problem ... Our military is also maintaining a close watch on any cyber attacks," South Korea's defence ministry spokesman Kim Min-Seok said Monday.
The drill comes at a time of heightened concern about cyber crime after a crippling attack against Sony Pictures which both Seoul and Washington have blamed on North Korea.
President Barack Obama is considering adding North Korea to the US list of state sponsors of terrorism after the attack, which prompted Sony to halt the release of comedy action film "The Interview".
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 and media agencies last year.
North Korea has denied involvement in the hacking, and threatened Sunday to hit back at the White House if it is targeted with sanctions.
It accused the Obama administration of being "deeply involved" in the making of "The Interview", which concerns a fictional CIA plot to kill Pyongyang's leader Kim Jong-Un.
Pyongyang also denied on Sunday that it had ever launched a cyber-attack against South Korea.
In recent years, hackers have deployed cyber-attacks on South Korean military institutions, commercial banks, government agencies, TV broadcasters and media websites.
South Korean officials say the North is believed to run an elite cyber war unit of at least 3,000 personnel.
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