North Korean hackers have the ability to hack, attack and even kill, a high-profile defector has said.

Kim Heung-Kwang told BBC Click that trained military hackers in North Korea number 6,000 and they are all capable of destroying infrastructure and killing people.

Martyn Williams, a journalist and Korean technology expert, told the BBC that the threat is "theoretical," but in light of last year's Sony Pictures hack attributed to North Korea, Kim said he believes North Korea is capable of more severe attacks. Kim is calling for international organizations to intervene.

"This issue can't be solved by one or two countries," Kim said. "The international community needs to pay attention to North Korea's attempts to destroy the internet."

Before Kim escaped North Korea in 2004, he taught computer science at Hamheung Computer Technology University. He did not teach hacking, according to BBC News, but he said his former students are part of Bureau 121. As HNGN previously reported, Bureau 121 is the no-longer-secret North Korean hacking unit that is believed to operate out of China.

South Korean power plants and banks have been the main targets of Bureau 121's attacks. "The size of the cyber-attack agency has increased significantly, and now has approximately 6,000 people," Kim told the BBC.

Kim estimates that 10 to 20 percent of North Korea's military budget is being spent on cyberwarfare. "The reason North Korea has been harassing other countries is to demonstrate that North Korea has cyber war capacity," he said. "Their cyber-attacks could have similar impacts as military attacks, killing people and destroying cities."

Williams 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."