North Korean spies have been operating in Malaysia, Singapore and Indonesia over the last two decades.
According to an intelligence source, their operation in the three countries is believed to be one of the biggest networks outside North Korea, with Malaysia and Singapore being the preferred choice of the Reconnaissance General Bureau (RGB).
The RGB is North Korea's intelligence agency which manages the country's clandestine operations.
To blend in and disguise themselves in Malaysia and Singapore, the highly qualified spies would double-up as engineers and technical consultants in the construction industry, and also operate Korean restaurants.
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"They used the restaurants as a main front to conduct intelligence gathering and surveillance, targeting Japanese and South Korean politicians, diplomats, top corporate figures and businessmen who visited or were based in these countries," the source said.
The RGB comes under the jurisdiction of the North Korean State Security Department or State Security Ministry and reports directly to the supreme leader Kim Jong-un.
In Indonesia, the source said, RGB was also known to be operating textile factories in major Indonesian cities, including Jakarta.
"One particular outfit located above a North Korean restaurant in central Jakarta was part of the RGB Indonesia office," said the source.
To finance its elaborate network, the source said the RGB had also been involved in drug smuggling.
This came to light after it failed to smuggle 125kg of heroin into Australia via the Pong Su, a commercial ship, in 2003.
The source said investigations by Australian police revealed the RGB had used Port Klang as a transit point and refitted a consignment of freezers, by removing the insulation, to hide the drugs.
The drugs were linked to RGB operatives.
The source said the RGB had also used Malaysia in the early 2000s as one of its destinations to reroute prohibited chemicals, which can be used to make nerve gas, to Pyongyang via China.