A ruling party lawmaker said Tuesday that North Korean hackers are believed to have stolen a large amount of important military secrets from a South Korean data center in September last year. 

Democratic Party Representative Rhee Cheol-hee said that 235 gigabytes of military data were leaked at the time from the Defense Integrated Data Center including the military's intranet and its external Internet network.

Rhee said the leaked data included around 270 counts of level two or level three classified information.

The secret files include the latest South Korea-U.S. wartime operational plans 5015 and 3100. 

OPLAN 5015 outlines how to handle an all-out war with Pyongyang and contains procedures to "decapitate" the North Korean leadership while OPLAN 3100 is a plan of response against the North's infiltrations and localized provocations.

The lawmaker said that also among the stolen files were contingency plans for South Korean special forces, reports made to top commanders of South Korea and the U.S., conference materials on the Ulchi Freedom Guardian exercise as well as information on military bases and power plants.