North Korea Cyber Army Hacked U.S.-South Korea Plans to 'Decapitate' Kim Jong Un, Seoul Lawmaker Says

North Korean leader Kim Jong Un gives field guidance at the Sci-Tech Complex, in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang October 28, 2015. North Korean hackers have reportedly stolen secret U.S.-South Korean military plans. KCNA/via Reuters

A cache of secret U.S.-South Korean military files including plans to taken down North Korean leader Kim Jong Un could have been stolen by hackers last year, a lawmaker for South Korea's ruling Democratic Party said Tuesday.

Rhee Cheol-hee said hackers broke into the Defense Integrated Data Center in September 2016 and stole a number of classified documents, citing defense ministry officials.

The news first emerged in April, with local newspaper Chosun Ilbo quoting anonymous defense ministry sources and noting that the ministry had previously downplayed the seriousness of the hacking.

According to Rhee, the ministry still has to identify the content of about 80 percent of the 235 gigabytes of data that was stolen.

"The Ministry of National Defense has yet to find out about the content of 182 gigabytes of the total (stolen) data," the lawmaker said in a statement quoted in the South Korean news agency Yonhap.

He also said that among the stolen files were Operation Plans 5015 and 3100. The operation plans are classified to the point that South Korean lawmakers from both ruling and opposition parties protested about the superficial briefing received by defence officials when they were introduced in 2015, as reported in the local press at the time.

It later emerged that OPLAN 5015 includes a pre-emptive strike on the North's nuclear and missile facilities, as well as "decapitation attacks" against Kim and the rest of the North Korean leadership.

While OPLAN 5015 is seen as a blueprint for a limited war, OPLAN 3100 instead deals with Seoul's response to possible North Korean localized provocation or commando infiltration.

According to Rhee, the hackers have also gained infortmation about state-of-the-art military facilities, power plants and the joint military drills with the U.S. as well as reports meant for U.S. commanders.

North Korea has denied reports of the hacking, but it is no secret Pyongyang has trained a group of elite hackers over the years. In 2014, a North Korean defector spoke of his experience at the country's military college for computer science where top students are hand-picked to join the cyber warfare cell. Cyber security experts also believe Pyongyang hackers may have been behind the global "WannaCry" ransomware global cyber attack in May.

In 2016, South Korea said more than 140,000 computers at 160 south Korean firms and government agencies had been targeted by North Korean hackers. Among the stolen files were military documents related to blueprints for the wings of F-15 fighters, Reuters reported.