South Korea has heightened its security readiness against cyber attacks as concerns grew over a series of online information leaks at nuclear power plants, officials said Tuesday.

Seoul's defence ministry said its cyber warfare unit increased its watch level against attacks from North Korean and other hackers, after the publication last week of a variety of information about the South's nuclear power plant operator on Twitter.

The 400-member unit was set up in 2010 as part of efforts to expand South Korea's cyber warfare capability to counter hacking threats from North Korea.

"In line with the heightened status of our national cyber security, the military has stepped up its readiness," defence ministry spokesman Kim Min-Seok told reporters.

The energy ministry, which is in charge of nuclear power plants, and other related state bodies have also beefed up cyber security.

The information posted on Twitter included designs and manuals for two reactors, as well as personal information on some 10,000 workers at Korea Hydro and Nuclear Power (KHNP), which operates 23 nuclear reactors and supplies about 30 percent of the country's electricity.

On Sunday the hacker suspected of being behind the leaks threatened to release more information unless the government shut down three reactors from December 25 and warned residents living near the plants to stay away for the coming months.

KHNP said the leaks would not affect the safety of its reactors but it nevertheless launched a two-day drill from Monday to test its ability to thwart a cyber attack.

More information was disclosed Tuesday, including what appeared to be diagrams of the nuclear reactors in Gori and Wolsong, southeast of Seoul, with a message scoffing at the drill.

South Korean President Park Geun-Hye urged her cabinet on Tuesday to beef up cyber security for nuclear power plants and other key infrastructure facilities.

"We must maintain a tight defence against cyber terrorism targeting nuclear power plants and other key facilities," she said.

Information leaks at nuclear power plants were serious crimes affecting "public safety", Park said, calling for a "thorough" investigation into who was responsible.