North Korean hackers allegedly stole classified military documents from a South Korean Defense Ministry database in September 2016, according to Rhee Cheol-hee, a member of South Korea's National Assembly, CNN reported Wednesday.

Rhee told CNN on Tuesday that the documents stolen included the South Korea-US wartime operational plan and a document that includes procedures to "decapitate" the North Korean leadership.

About 235 gigabytes worth of military data was stolen by the hackers, Rhee told CNN.

A spokesman from South Korea's Defense Ministry declined to comment, saying the information is classified.

The Pentagon also declined to comment specifically on reports of the potential breach, but spokesman Col. Robert Manning said on Tuesday that the US is "confident in the security of our operations plans and our ability to deal with any threat from North Korea."

CNN reported that depending on the level of detail in the stolen plans, the hack could pose serious challenges for the US-South Korean alliance.

"If the North Koreans in fact accessed the US/South Korean defense plans, this is a treasure trove of information and presents a real danger," said CNN military analyst and retired Lt. Col. Rick Francona.

"If I had access to the enemy's plans, not only would I know what forces were going to be arrayed against me, I would know where they will be, what weapons they will have, where the command and control nodes will be established -- all critical war-fighting information."

Meanwhile this information emerged around the same time the US military flew two strategic bombers over the Korean peninsula in a show of force late on Tuesday.

According to Reuters, North Korea has launched two missiles over Japan and conducted its sixth nuclear test in recent weeks as it fast advances toward its goal of developing a nuclear-tipped missile capable of hitting the U.S. mainland.

The two US Air Force B-1B bombers were joined by two F-15K fighters from the South Korean military after leaving their base in Guam, South Korea’s Joint Chiefs of Staff said in a statement on Wednesday.

After entering South Korean airspace, the two bombers carried out air-to-ground missile drills in waters off the east coast of South Korea, then flew over the South to waters between it and China to repeat the drill, the release said.

Reuters reported that the US military said in a separate statement it conducted drills with Japanese fighters after the exercise with South Korea, making it the first time US bombers have conducted training with fighters from both Japan and South Korea at night.