The South Korean paper Dong-A Ilbo, citing opposition party lawmaker Kyeong Dae-soo, claims that 60 classified military documents were among 40,000 hacked from South Korea’s Daewoo Shipbuilding and Marine Engineering Co. in April 2016.
South Korea’s Aegis-equipped ships and submarines are key to plans for a preemptive strike against North Korea.
They are equipped with ballistic missiles to target key facilities in the South.
The documents include information on construction technology, blueprints, weapons systems and evaluation of the ships and submarines.
Mr Kyeong told the newspaper that he was briefed on the hack from South Korea’s defence ministry.
North Korea has been developing cyber capabilities as trade sanctions and a poor economy have made it hard to invest in conventional military capabilities.
The rogue nation has been accused of hacking everything from Sony Corp to bitcoin and other cryptocurrencies.
It was even accused of hacking the NHS, which it denies.
The official Korean Central News Agency said: “We made it very clear that the DPRK had no connection at all with acts of cybercrime.”
The claims come after it was reported earlier this month that North Korean hackers have stolen the USA’s hyper-classified plans for war on the peninsula.
The data even includes a plot to “decapitate” Kim Jong-un.
Rhee Cheol-hee, a South Korean lawmaker, said North Korea now has its hands on the latest South Korea-US wartime plans, the South’s Yonhap News Agency reported.
Pyongyang has denied responsibility for the cyber attack, scolding Seoul for “fabricating” claims of online attacks.
The stolen plans coincide with the North ramping up its rhetoric and testing six missiles this year, including a nuclear missile which the North claims could reach the US west coast.
Russian lawmaker Anton Morozov visited Pyongyang earlier this month.
He warned the North possesses a ballistic missile which will be able to reach US territory after it is modernised.
Mr Morozov said North Korea is aiming to increase the range of its ballistic missiles from 3,000km to 9,000km – which could reach the US Pacific island of Guam.
Mr Morozov, who embarked on a three-day visit to the despot kingdom with two other Russian lawmakers, said on Friday Kim was preparing to launch another long-range missile “in the nearest future”.