North Korea hacked into more than 140,000 computers at large South Korean conglomerates and government agencies and planted malicious codes that may have been intended for a massive cyber attack that has been thwarted.
The hacking originated from an internet address traced to the North Korean capital and targeted a software used by about 160 companies and government agencies to manage their computer networks, Yonhap news agency reported on Monday, citing the police.
The internet address was identical to the one used in a 2013 cyber attack against South Korean banks and broadcasters that froze their computer systems for more than a week.
South Korea blamed the North for that attack.
The South Korean police agency's cyber investigation unit uncovered the hacking and worked with the companies and agencies affected to neutralise the malicious codes and prevent them from being used in a large-scale cyber attack, Yonhap said.
The police's cyber investigation unit could not immediately confirm the report.
South Korea has been on heightened alert against the threat of cyber attacks by North Korea after it conducted a nuclear test in January and a long-range rocket launch last month that led to new UN sanctions.
In March, the South's spy agency said it had intercepted an attempt to hack into the South's computer networks to attack the transport system's control network, blaming the North for the attempt.
North Korea has worked for years to develop the ability to disrupt or destroy computer systems that control public services such as telecommunications and other utilities, according to a North Korean defector familiar with the effort.
The United States accused North Korea of a cyber attack against Sony Pictures in 2014 that led to the studio cancelling the release of a comedy based on the fictional assassination of the country's leader, Kim Jong Un.
North Korea denied the accusation.