South Korea's government on Tuesday accused North Korea of carrying out cyber attacks last December on its nuclear power plant operator, describing them as a provocation which threatened people's lives and safety.
"It's a clear provocation against our security," the unification ministry said after investigators concluded that the North was behind the attacks.
"We condemn North Korea's persistent cyber-terror targeting our country and the international community," the ministry said in a statement, accusing Pyongyang of "taking the life and safety of our people as a hostage".
The South's accusations come at a time of elevated tensions with the North, which has condemned an ongoing US-South Korean military drill as a rehearsal for invasion.
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, Korea Hydro and Nuclear Power (KHNP).
The leaks prompted the South to heighten cyber-security and form an investigation team involving experts, government officials and state prosecutors.
The team earlier on Tuesday said the hackers intended to cause a malfunction at atomic reactors but failed to break into their control system.
It said malicious codes used in the cyber-attacks were similar to those which North Korean hackers have employed before.
"We've reached the conclusion that the crime was committed by a group of North Korean hackers seeking to stir up social unrest and agitation in our country," the investigators said in a statement.
The team said the hackers using multiple Internet protocol addresses based in China had sent some 6,000 "phishing" emails to 3,571 former and current KHNP workers to steal the data.
KHNP officials have said the 23 nuclear reactors, which supply about 30 percent of the country's electricity, were safe because their control system was separated from external networks.
They also said the material leaked by the hackers was not classified and did not affect safety.
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 last year which damaged the computer network of Sony's Hollywood film unit over its controversial North Korea-themed satirical film "The Interview".
Pyongyang denied involvement in the hack of Sony but strongly condemned the film, which features a fictional plot to assassinate leader Kim Jong-Un.