The Pentagon, responding to reports that North Korean hackers have stolen secret war plans in a cyberattack on a South Korean defense center, said Tuesday it remains confident the plans are secure.

"We've seen the media reporting on last year's potential breach of the ROK-U.S. alliance plans related to defending the Korean Peninsula," said Col. Rob Manning, a Pentagon spokesman. "And although I will not comment on intelligence matters or specific instances related to cyber intrusion, I can assure you that we are confident in the security of our operations plans and with our ability to deal with any threat from North Korea."

Related: Top US ground commander all but declares victory over ISIS in Iraq

The report came from a member of South Korea's ruling party, who said hackers managed to steal a large cache of documents in September last year, including a summary of a U.S. and South Korean plan for the defense of the south, known as OPLAN 5027.

The South Korean lawmaker, Rhee Cheol-hee, said some 235 gigabytes of military documents had been stolen from the Defense Integrated Data Centre, and that 80 percent of them have yet to be identified, according to the BBC.

South Korean media also reported the stolen documents included a contingency plan for a so-called "decapitation" attack to assassinate North Korean leader Kim Jong Un.

Sign up for the Daily on Defense newsletter

The Guardian newspaper reported the data breach may have resulted from a South Korean officer using an unsecured USB memory stick to download it.

According to the South Korean newspaper Chosun Ilbo, one document outlines potential troop deployments, lists North Korean targets, and describes amphibious landing scenarios as well as plans for a post-war occupation.

Yonhap News Agency also said the plan called for the deployment of 700,000 U.S. troops in the event of a full-scale war.

A spokesman for the South Korean defense ministry insisted the document was merely an 11-page summary and did not contain sensitive information.