South Korean lawmaker Rhee Cheol-hee said the information was from his country’s defense ministry.
The compromised documents include wartime contingency plans drawn up by the United States and South Korea.
The documents also include reports to the allies’ senior commanders.
The South Korean defense ministry has so far refused to comment about the allegation.
Plans for South Korea’s special forces were reportedly accessed, along with information on significant power plants and military facilities in the South.
Rhee Cheol-hee belongs to South Korea’s ruling party, and sits on its parliament’s defense committee. He said some 235 Gb of military documents had been stolen from the Defense Integrated Data Centre, and that 80% of them have yet to be identified.
The hack took place in September 2016. In May 2017, South Korea said a large amount of data had been stolen and that North Korea may have instigated the cyber attack – but gave no details of what was taken.
According to South Korea’s Yonhap news agency, Seoul has been subject to a barrage of cyber attacks by its North Korea in recent years, with many targeting government websites and facilities.
North Korea is believed to have specially-trained hackers based overseas, including in China.
Pyonyang has accused South Korea of “fabricating” the claims.
News that North Korea is likely to have accessed the Seoul-Washington plans for all-out war in the Koreas will do nothing to soothe tensions between the US and North Korea.
The US and North Korea have been at verbal loggerheads over Pyongyang’s nuclear activities, with the US pressing for a halt to missile tests and the North vowing to continue them.
North Korea recently claimed to have successfully tested a miniaturized hydrogen bomb, which could be loaded onto a long-range missile.
In a speech at the UN last month, President Donald Trump threatened to destroy North Korea if it menaced the US or its allies, and said its leader “is on a suicide mission”.
Kim Jong-un responded with a rare statement, vowing to “tame the mentally deranged US dotard with fire”.
President Trump’s latest comment took the form of a cryptic tweet at the weekend, where he warned that “only one thing will work” in dealing with North Korea, after years of talks had proved fruitless. The president did not elaborate further.
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