F-15 fighter jet wing blueprints among defence files stolen during North Korea's attack on140,000 computers belonging to both private firms and state agencies

North Korea hacked into more than 140,000 computers at 160 South Korean firms and government agencies, planting malicious code under a long-term plan laying groundwork for a massive cyberattack against its rival, police in the South said on Monday.

South Korea has been on heightened alert against cyber attacks by the North after Pyongyang conducted a nuclear test in January and a long-range rocket launch in February that led to new UN sanctions.

The North has always denied wrongdoing.

The hacking began in 2014 and was detected in February, after North Korea managed to steal information from two conglomerates including defence-related material, South Korea’s police cyber investigation unit said.

“There is a high possibility that the North aimed to cause confusion on a national scale by launching a simultaneous attack after securing many targets of cyber-terror, or intended to continuously steal industrial and military secrets,” it said.

The hackers took no action after gaining control of servers and computers at some corporate groups and waited, as they continued to hack into more targets in what police said was likely an effort to build the scale of a planned attack.