North Korean hackers are capable of attacks that could destroy critical infrastructure and even kill people, a high-profile defector has warned. Speaking exclusively to BBC Click, Prof Kim Heung-Kwang said the country had around 6,000 trained military hackers. The warning follows last year’s Sony Pictures hack - an attack attributed to North Korea. Korean technology expert Martyn Williams stressed the threat was only ‘theoretical’. Prof Kim has called for international organisations to step in to prevent North Korea launching more severe attacks. For 20 years Prof Kim taught computer science at Hamheung Computer Technology University, before escaping the country in 2004.

While Prof Kim did not teach hacking techniques, his former students have gone on to form North Korea’s notorious hacking unit Bureau 121.

The bureau, which is widely believed to operate out of China, has been credited for numerous hacks. Many of the attacks are said to have been aimed specifically at South Korean infrastructure, such as power plants and banks. Speaking at a location just outside the South Korean capital, Prof Kim told the BBC he has regular contact with key figures within the country who have intimate knowledge of the military’s cyber operation. ‘The size of the cyber-attack agency has increased significantly, and now has approximately 6,000 people,’ he said.

‘The reason North Korea has been harassing other countries is to demonstrate that North Korea has cyber war capacity,’ he added. ‘Their cyber-attacks could have similar impacts as military attacks, killing people and destroying cities.’ Speaking more specifically, Prof Kim said North Korea was building its own malware based on Stuxnet - a hack attack, widely attributed to the US and Israel, which struck Iranian nuclear centrifuges before being discovered in 2010.

‘A Stuxnet-style attack designed to destroy a city has been prepared by North Korea and is a feasible threat,’ Prof Kim said. Earlier this year, the South Korean government blamed North Korea for a hack on the country’s Hydro and Nuclear Power Plant. ‘Although the nuclear plant was not compromised by the attack, if the computer system controlling the nuclear reactor was compromised, the consequences could be unimaginably severe and cause extensive casualties,’ Prof Kim said.

Martyn Williams is a journalist who follows closely the development of technology in North Korea. He told the BBC: ‘I think it’s important to underline that this is theoretical and possible from non-North Korean hackers too. ‘It’s conceivable that hackers would try something and lives could be at risk. He noted an attack in 2013 on South Korean broadcasters, which he said was ‘an attempt to throw the country into confusion’.

‘If TV had gone off air and then ATMs stopped working, people might have panicked.’ When it comes to cyber-attacks, few groups are as notorious as North Korea’s Bureau 121, which has operated since the late nineties. Most security researchers agree that the group operates out of China. Specifically, in the basement of a restaurant, rated highly on TripAdvisor for its tremendous Korean food.