North Korean leader Kim Jong-un, right, inspects the Pukjung Machine Complex, one of North Korea's major manufacturing facilities, in this photo released by the state-run Korean Central News Agency, Sept. 3. Yonhap

North Korea has carried out intensive cyber attacks against South Korean shipbuilders in an effort to bolster its military capabilities upon leader Kim Jong-un's instruction, Seoul's spy agency said Wednesday.

The National Intelligence Service (NIS) said it had detected "multiple" attempts at cyber offenses by North Korean hacking groups against domestic shipbuilding companies and their employees in August and September.

The hackers infiltrated computers of IT maintenance companies for shipbuilders or distributed malicious emails to the employees to steal sensitive information, according to the agency.

"Such intensive attacks are presumed to have been made as Kim Jong-un ordered to build mid- to large-sized warships," the agency said. "North Korea is expected to continue such attacks against our shipbuilders and component manufacturing firms."

In response, the NIS vowed to beef up monitoring and extend supportive measures for domestic companies, and called on them to strengthen security checks and other preventive measures.

The North Korean leader has recently called for bolstering his country's military power and the development of its shipbuilding industry.

While inspecting the Pukjung Machine Complex last month, Kim reaffirmed that a key party meeting "would set forth an important line of the modernization of the complex and the development orientation of the shipbuilding industry of the country," the North's Korean Central News Agency said. (Yonhap)