A South Korean lawmaker says North Korean hackers broke into a shipyard and stole plans for naval technologies as Pyongyang seeks its own submarine fleet armed with nuclear missiles.

Kyeong Dae-soo, a lawmaker from South Korea's hawkish Liberty Korea Party, made public the claim that North Korea stole the plans less than a month after a "ridiculous mistake" allowed the US and South Korea's war plans to be hacked by Pyongyang.

"We are almost 100 percent certain that North Korean hackers were behind the hacking and stole the company's sensitive documents," Kyeong told Reuters. Defense industry officials corroborated Kyeong's story to The Wall Street Journal.

Kim Jong Un with North Koreas just after the test fire of a surface to surface medium long range missile.(image KCNA)

Sixty "classified documents including blueprints and technical data for submarines and vessels equipped with Aegis weapon systems" made their way into North Korean hands, according to The Journal.

The news of the theft comes as US intelligence assesses that North Korea has begun construction of a new class of 2,000-ton submarine — likely the largest ever attempted by the small country, The Diplomat reports. The submarine appears to follow North Korea's tradition of attempting to field an undersea leg of its nuclear deterrent, mimicking the US.

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Basically, by deploying nuclear weapons on land and at sea, North Korea makes it nearly impossible for any one attack from the US or any other adversary to remove its nuclear capabilities.

Kyeong said that the information hacked also contained details on submarine-launched ballistic missiles, which North Korea has tried and failed to perfect in the past.

Though the US and South Korea enjoy a massive edge in submarine technology over North Korea, the shallow coastal waters around the Korean Peninsula are noisy with irregular currents, meaning even the best submarine hunters might struggle to hunt down and destroy their targets. North Korea is thought to operate about 60 submarines, but none of those can likely launch a ballistic missile yet.

Additionally, Aegis technology, also leaked in the hack, is used by the US and its allies to fend off incoming missiles or missiles fired overhead, like North Korea's frequent long-range missile tests.

Though North Korea likely can't duplicate the technology, Aegis is the world's most advanced at-sea missile defense, and any leaks could compromise the safety of the US Navy.

Earlier in October, the news came out that North Korea had hacked the US and South Korea's war plan by exploiting a lapse in security. Experts estimate that the cyber threat from North Korea is growing and could seriously complicate any conflict.