South Korea's intelligence agency says North Korean hackers have carried out cyberattacks against South Korean shipbuilders to obtain information needed to build warships.

The National Intelligence Service on Wednesday disclosed multiple South Korean shipbuilders came under intensive cyberattacks from North Korean hacking groups in August and September.

The agency said phishing emails were sent to employees to steal sensitive information.

An agency probe found that North Korean leader Kim Jong Un ordered that warships be built, and the cyberattacks were carried out in response.

Kim last month attended the launching ceremony for a newly built "tactical nuclear attack submarine" and inspected a state-run enterprise related to shipbuilding, where he expressed resolve to bolster the military prowess of North Korea's navy.

The agency says the cyberattacks will likely continue. It has warned shipbuilders and shipbuilding parts makers to beef up their online security.