The North's cyber war unit now has 5,900 personnel, compared with 3,000 two years ago, the South's Yonhap news agency said. (Agencies)
"The communist country operates a hacking unit under its General Bureau of Reconnaissance, which is home to some 1,200 professional hackers," a military source was quoted as saying.
North Korean hackers have launched cyber attacks through overseas bases in countries such as China, the source said.
In recent years, hackers have used malware deployments and virus-carrying emails for cyber attacks on South Korean military institutions, commercial banks, government agencies, TV broadcasters and media websites.
Investigations into past large-scale cyber assaults have concluded that they originated in North Korea.
The North has denied any involvement and accuses Seoul of fabricating the incidents to fan cross-border tensions.
South Korea has increased its Internet security budget to train experts since it set up a special cyber command in 2010, amid growing concern over its vulnerability.
The North's cyber war unit now has 5,900 personnel, compared with 3,000 two years ago, the South's Yonhap news agency said.