After a suspected hacker leaked a series of online information from the computer systems of a nuclear plant operator, South Korea is now conducting cyber attack drills at four nuclear power plant complexes for two days.

"A two-day drill is under way through simulators to ensure the safety of our nuclear power plants under cyber attacks," said a spokesman for the Korea Hydro and Nuclear Power Co (KHNP). "We are making utmost efforts, working closely with the government to assess the data leak at certain nuclear power plants, which adds to social unease."

According to KHNP, the hackers stole only "non-critical" data and that the nuclear installations, including the 23 atomic reactors of the country, were not threatened. Officials also added that they were confident in fending off any breach to the atomic facilities.

"The control system is designed in such a way [that] there is no risk whatsoever," said Chung Yang-ho, deputy energy minister.

The hacker, who posted about the attack on social media on Dec. 19, used an account that bears the name "president of the antinuclear reactor group." The message, along with the leaked documents, was supposed to appear as though they were sent from a location in Hawaii.

The unidentified hacker threatened to leak more files on the web unless the government closed down three reactors, setting the deadline on Christmas day. There was also a warning given to the people living near the reactors to avoid the area in the next couple of months.

"If you keep this up, we'll give the world the rest of the 100,000 documents we still have. How would you like that?" the hacker tweeted.

The group leaked the documents through a cloud-based file-sharing service. Some of the documents include the program manual that was used at the nuclear reactors, a valve diagram at Wolseong Reactor No. 1 and a cooling system diagram for Kori Reactor No. 2. There was also leaked information on 10,000 employees of KHNP.

The Twitter post also asked for money in exchange for the data.

South Korea's state prosecutors have launched an investigation on the attack but have been unsuccessful in finding out the identity of the perpetrator.

The two-day precautionary exercises came a few days after the U.S. declared Pyongyang's involvement in the Sony Pictures hack. As a result, the company decided to cancel the release of the film, which portrays the fictional assassination attempt on the leader of North Korea.