Computer hackers accessed a secret US-South Korean military plan to defend the Korean Peninsula if war breaks out, South Korea's military said, dpa reported.
The hackers gained access in November to Operation Plan 5027, which calls for sending about 700,000 US troops to Korea if a full-scale war begins, the South Korean military said.
It said hackers stole 11 PowerPoint slide pages that are part of Operation Plan 5027.
The military said the breach occurred after a military officer failed to remove a USB device from a military computer when he changed from using a restricted intranet to the internet.
The officer is facing punishment, the military said.
The internet address the hackers used was traced back to an IP location in China and South Korea believes North Korea might have been involved in the hacking attack.
The military has been trying to trace the hackers but without producing any tangible evidence to prove that North Korea was involved in the attack.
In May, media reports from Seoul said North Korea was maintaining a computer-hacker network working on attacks against the South Korean and US militaries.
Media reports have said there are at least 500 professional hackers in Pyongyang.