Shenyang , China ( CNN ) On the streets of the neon-lit Chinese city of Shenyang , you 'll find a restaurant , hotel , and other businesses owned and operated by the North Korean government . You 'll also find a secret network of North Korean hackers , known as Bureau 121 , according to defector Kim Heung-Kwang . `` It 's easy for them to work secretly . It also has great Internet infrastructure , '' says Kim Heung-kwang , a former Pyongyang computer science professor who escaped North Korea in 2004 . Kim says some of his own students became cyber warriors for the hacker network . `` By day , they worked regular jobs . But the rest of the time , they were acting on orders from Pyongyang , '' he says . RELATED : North Korea pushes back against U.S. sanctions for Sony hack . What is 'Bureau 121 ' ? Kim claims North Korean hackers operated secretly in Shenyang for years , moving from location to location to conceal their whereabouts and activities . `` Bureau 121 began its large-scale operation in China in 2005 . It was established in the late 90s , '' Kim says . `` Team members entered China separately -- in smaller groups -- 20 members at a time , '' he says . `` When they entered China , they came under different titles . For example an office worker , an official with a trade company or even as a diplomatic staffer . '' Long before North Korea had its own Internet , it dialed in to servers in Shenyang , in Liaoning Province , in the country 's north . Today , nearly all of North Korea 's Internet traffic is still routed through China . Kim says the operation in China scaled back considerably a few years ago , when North Korea expanded its high speed Internet access . But he believes hackers are still operating in Shenyang . `` North Korea does have illicit activities in China , '' says Steve Sin , a terrorism expert at the University of Maryland and former U.S. military intelligence analyst . Sin wrote a report naming the northeastern Chinese city of Shenyang as a North Korean hacker hub . `` It has the location , security , as well as infrastructure , '' Sin says . `` Right now , the best information available to us is that they are still conducting such an operation and they can still conduct such an operation from that location . '' The North Korean embassy in Beijing , and government officials reached by email in Pyongyang , tell CNN they have no comment on Kim 's claims . For its part , China says it opposes any illegal cyber activity on its territory . Inside Shenyang . For a Chinese city , Shenyang has a distinctly North Korean flavor . At the state-owned `` Pyongyang Restaurant , '' waitresses told us they came to China on what is considered a prestigious three-year assignment . They say they 're all from the same university in Pyongyang . They serve `` North Korean meals , '' in far more substantial portions than the food rations at home . They also sing and entertain customers with popular folks songs like `` Arirang , '' considered the `` unofficial anthem '' for both North and South Korea . A short drive away is state-owned The Chilbosan hotel , a joint venture between North Korea and China . The women who work in the hotel wear traditional North Korean clothing . There is an art gallery featuring pieces that glorify North Korea , a gift shop with Pyongyang souvenirs , and even a travel office for North Korea 's state-owned national airline `` Air Koryo . '' There is also a marketplace -- which caters to Shenyang 's large Korean community . And not far from the government business , there 's a North Korean consulate . As the biggest Chinese city near North Korea , Shenyang is a place where many from Pyongyang come to work . According to Sin , they also come to hack . `` If you are going to conduct illicit activities or covert operations , it 's better to hide among this population , if you will , '' Sin says . `` Large , complicated attacks require a certain amount of cyber infrastructure to carry it out . Shenyang , China has that capability . ''
Underground groups of North Korean hackers are operating in China , defector says . They 're based in Beijing , he says , near the North Korean embassy . North Korean officials in Beijing and other government officials declined to comment .