North Korea has roughly doubled the number of hackers it employs to conduct cyber-attacks, South Korea’s Yonhap News said on Sunday.

The news agency quoted an unidentified military source as saying North Korea “appears to have” 5,900 personnel for cyber-warfare, up from about 3,000 people two years ago.

Yonhap didn’t disclose how its source had access to the current information.

The South Korean government often uses anonymous leaks to put intelligence regarding North Korea into the public domain. It sometimes uses similar leaks to disclose false or unverified information for political purposes.

The 3,000-person figure was provided at a cyber-security conference in Seoul in 2012 by Kim Heung-kwang, a former professor at Pyongyang Computer Technology University and member of the North Korea Intellectuals Solidarity group.

The same source told Yonhap that North Korea’s \Reconnaissance General Bureau employs around 1,200 hackers. The bureau is home to the country’s spying activities and was previously blamed by Seoul for several attacks on South Korean computer networks.

Earlier this year, the U.S. Department of Defense said North Korea might be building up its cyber attack force in preference of conventional forces because of the lower cost involved.

“Given North Korea’s bleak economic outlook, [offensive cyber operations] may be seen as a cost-effective way to develop asymmetric, deniable military options,” the DOD said in the annual report to Congress on “Military and Security Developments Involving the DPRK.”