South Korean prosecutors on Tuesday blamed North Korea for cyber attacks against the country's nuclear reactor operator last December following a probe into Internet addresses used in the hacking.

The conclusion comes less than a week after a hacker believed to be behind the cyber attacks on Korea Hydro and Nuclear Power Co. Ltd released more files on Twitter that are believed to have been taken in December. The investigation included last week's leak of a blueprint and test data.

"The malicious codes used for the nuclear operator hacking were the same in composition and working methods as the so-called kimsuky that North Korean hackers use," a statement from Seoul central prosecutors' office said.