The documents reportedly include contingency plans for war, drawn up by the US and South Korea, along with details on power plants and military facilities. -South Korean wartime "decapitation strike" plans against the North, according to South Korean media reports. Operational Plan 3100 details how to respond to North Korean provocations, Yonhap reports.
The South Korean Defense Ministry has not commented on the allegations.
It came days after he said that diplomatic efforts with North Korea have consistently failed, adding that "only one thing will work".
North Korea warned last month it could shoot down US bombers at its discretion amid heightened tensions in the wake of Pyongyang's sixth ever nuclear test.
In a pair of tweets sent Saturday afternoon, Trump said past agreements with North Korea have all been violated.
Two B-1B strategic bombers of the US Air Force trained with South Korea's F-15K fighter jets over and near the Korean peninsula in yet another show of force against North Korea, defence authorities here said on Wednesday.
France Says Won't Recognize France Catalonia's Unilateral Secession from Spain
Catalonia's Parliament had been due to meet on Monday with the expectation that it would endorse the declaration of independence . It said its priority was to protect "clients, shareholders and employees" given the current political and social situation.
A pre-emptive strike against Pyongyang's leadership would be hard to undertake, but it's widely seen as the most realistic of the limited military options Seoul has to deny a nuclear attack from its rival. (STR/AFP/Getty Images) North Korean dictator Kim Jong-Un and other communist officials at an undisclosed location in North Korea in this picture released by North Korea's official Korean Central News Agency.
North Korea's nuclear weapons program has become the most urgent national security issue during the Trump presidency.
Lee said that 235 gigabytes of military documents were taken, but the military has yet to identify 80 percent of the documents that were compromised.
"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.
"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", Pentagon spokesman Colonel Robert Manning told reporters.
The report said Seoul has been hit by a number of cyber attacks in recent years trying to breach government and corporate websites and has since worked to bolster its cyber defense systems. North Korea routinely denies responsibility.