Computers at a nuclear power plant in South Korea have been compromised by a hacker, but the plant's operator says no critical data has been leaked.

The hacker was able to access blueprints, floor maps and other information on the plant, the South Korean Yonhap News Agency reported Sunday. Using a Twitter account called "president of anti-nuclear reactor group," the hacker has released a total of four postings of the leaked data since December 15, each one revealing internal designs and manuals of the Gori-2 and Wolsong-1 nuclear reactors run by Korea Hydro and Nuclear Power Co. (KHNP), Yonhap added. The hacker has threatened to leak further information unless the reactors are shut down.

KHNP has insisted that the leaked information is not critical and does not undermine the safety of the reactors. The company also played down the threat of any type of cyberattack, saying that the reactors' controllers are protected because they're not linked to any external networks, according to the Wall Street Journal.

The hacking against KHNP nuclear plants occurs in the midst of a major hack against Sony Pictures over its movie "The Interview," a comedy about an assassination attempt against North Korean leader Kim Jong-un. The FBI has accused North Korea of orchestrating the Sony hack, though the country has denied any involvement. As a further response, North Korea suggested a joint investigation into the hack with the US but then accused the US of being involved in the making of the film, according to The Guardian.

Despite the increased tension, no fingers have been pointed at North Korea for the hacking against the KHNP power plants. An official at KHNP told Reuters that the hacking appeared to be the work of "elements who want to cause social unrest," but added that he had no one specific in mind.

Government officials looking into the incident were able to trace the hacker's IP address to a PC located in a specific location, Yonhap said. Investigators have been sent to the location as well as to the plant's reactors to probe further.