North Korea on Thursday denied involvement in cyber-attacks on South Korea's nuclear power plant operator, accusing Seoul of fabricating a story to shift the blame for high cross-border tensions.

Last December hackers published designs, manuals and other information about South Korean reactors on Twitter, along with personal information about workers at their operating company.

The leaks prompted the South to heighten cyber-security and launch an investigation involving experts, government officials and state prosecutors.

Last week Seoul blasted Pyongyang for seeking to throw South Korea into "social chaos" with cyber-attacks on the reactors, describing them as a threat to people's lives and safety.

On Thursday Pyongyang's state Internet research institute insisted North Korea had never been involved in any cyber-attacks on the South.

"We never think about a bad act against the interests of our brethren under any circumstances," it said in a statement published by the official KCNA news agency, dismissing the South's claims as "sheer fabrication".

The charges that North Korea was behind the reactor hack reflect South Korea's "sinister" scheme to shift responsibility for strained inter-Korean relations, it said.

Tensions between the neighbours are running high after the South this month held joint military drills with the United States -- condemned by the North as provocative rehearsals for invasion.

Seoul has blamed North Korean hackers for a series of cyber-attacks on military institutions, banks, government agencies, TV broadcasters and media websites in recent years.

The United States also said the North was behind a cyber-attack on Sony over its controversial North Korea-themed satirical film "The Interview" last year.

Pyongyang denied involvement in the Sony hack but strongly condemned the film, which features a fictional plot to assassinate leader Kim Jong-Un.