The South Korean counter-espionage agency which launched a probe into an alleged hacking attack on a naval warship building firm last month says it believes North Korea may be behind the hack. On 20 April, Hanjin Heavy Industries & Construction Co, the largest naval shipbuilders in South Korea, was hit by a cyber-attack leaving possible classified files exposed.

"After identifying signs that Hanjin Heavy Industries may have been hacked on 20 April, the Defence Security Command is currently leading a security investigation. North Korea could have been involved, but we are not absolutely sure," an official source told Yonhap.

Hanjin has been known to build the latest frigates, landing ships, patrol vessels, high-speed boats and amphibious assault vessels including the ROKS Dokdo for the South Korean Navy. It is believed the company's intranet contained sensitive military information including sketches and operational manuals of some of these naval war machines. The source said the Command is currently checking if any such information was leaked as much of it is classified.

South Korea's military was the target of cyber-attacks in 2014 and 2015 with the North suspected in both cases. A North Korean defector told the BBC in May last year that Pyongyang allegedly maintains an army of 6,000 trained military hackers and the regime shells out up to 20% of its military budget on cyber training and operations.

The South has also started stepping up its defence against such attacks. It recently encouraged college students to participate in hackathons and prepare some of them for future counter-hacking programmes. The South Korean government had stated last September that its government institutions alone had suffered more than 114,000 cyber-attacks since 2011.