President Park Geun-hye called on officials Tuesday to beef up the cyber security of the country's nuclear reactors and other key infrastructure facilities following cyber-attacks in South Korea and the United States.

The call comes as South Korea is struggling to find those responsible for a series of leaks of information related to two nuclear reactors. Officials said the Gori and Wolsong nuclear power plants, both located more than 400 kilometers from the capital Seoul, remain safe as their innermost networks are completely cut off from the Internet, though the latest incident raised safety concerns.

She urged officials to "again fundamentally check the preparations against cyber terrorism on our nation's key facilities, including nuclear reactors," in a cabinet meeting in the new administrative city of Sejong, about 150 kilometers of Seoul.

She also said the recent cyber attack on Sony Pictures is the latest reminder of the seriousness of cyber terrorism.

The comments came as North Korea appeared to have lost access to the Internet amid a cyber stand-off with the U.S. over the hacking of Sony pictures.

Major Internet sites based in the communist nation had been down since 1 a.m. before being restored before midday. They included several sites run each by the Korean Central News Agency and the Rodong Sinmun, Pyongyang's two key mouthpieces.

Pyongyang's Internet blackout came after U.S. President Barack Obama vowed a "proportional" response to the recent hacking of Sony Pictures Entertainment blamed on North Korea. (Yonhap)