 -- Hackers in North Korea have allegedly stolen a cache of classified military documents from South Korea, according to a South Korean lawmaker.

Lee Cheol-hee, a member of South Korea's ruling Democratic Party, initially told local media outlets that the documents were taken in a September 2016 hack of the country's Defense Ministry. The ministry would not comment to ABC News, citing national security concerns.

"The way it got hacked was preposterous," Lee told ABC News, confirming what he initially told South Korean media. "It wasn't because North Koreans had advanced hacking skills but was due to negligence on the South Korean part."

He continued, "I don't think the situation is as serious as it may sound but the reason why I alerted this is to push the new administration and the Defense Ministry to quickly find remedies so that this kind of loss don't happen again."

The sensitive documents reportedly contained U.S.-South Korean plans for a decapitation strike against North Korea — removing its leadership — a plan that has reportedly angered dictator Kim Jong Un.

About 80 percent of the material stolen has not yet been identified. The stolen trove totals about 235 gigabytes of data, according to reports.

"We have seen the media reporting on last year’s potential breach of the Republic of Korea-U.S alliance plans related to defending Korean peninsula," said Col. Rob Manning, a Pentagon spokesman.

"Although I will not comment on intelligence matters or specific incidents related to cyber-intrusion, I can assure you that we are confident in the security of our operations plans and our ability to deal with any threat from North Korea," said Manning. "We’ll continue to work closely with our partners in the international community in identifying, tracking and countering any cyber threats.”

He added that "The ROK-U.S. alliance remains steadfast and their commitment to make sure they safeguard that information and ensure readiness on the Korean peninsula to counter any North Korean threats.”

This hack was originally reported last May, but this is the first time details of what was stolen have been provided.

North Korea has been linked to other hackings. Among the most notable targeted Sony Pictures in 2014 as it was releasing "The Interview," a comedy about an assassination attempt on Kim. More recently, the WannaCry ransomware attack targeted hospitals across the United Kingdom. North Korea has denied involvement in any of the hacks and attacks.

ABC News' Joohee Cho contributed to this report.