But in fact the hackers got their hands on OPLAN 5015, part of South Korea-U.S. combined operations in a full-scale war between the two Koreas, which was worked out in 2015.

Suspected North Korean hackers managed to break into the Defense Ministry's intranet in August and September last year, but South Korean authorities have previously claimed that nothing significant leaked out.

The Defense Ministry admitted the leak in documents submitted to Minjoo Party lawmaker Rhee Cheol-hee under a freedom-of-information request. The ministry admitted hackers "presumed to be North Koreans" stole 235 GB of documents by breaking into the military's Intranet and other networks.

But it only listed 53GB or about 10,700 documents, a mere 22.5 percent of all leaked documents. They include OPLAN 5015, the newest war plan that Seoul and Washington have worked; OPLAN 3100 for a response against commando infiltration or a local provocation; and a contingency plan for the Special Warfare Command in the event of a sudden change in the North or in response to a major provocation.

These plans are aimed at winning a war at an early stage by minimizing damage to South Korea and carrying out pinpoint decapitation operations against top North Korean leaders. "I can't reveal further details because they are a military secret," Rhee said.

The leaked documents also include personnel reports on key South Korean and U.S. military, and the minutes of meetings about South Korea-U.S. military drills, as well as data on military installations and power plants in South Korea.