Prosecutors on Thursday indicted an alleged North Korean spy suspected of impersonating a defector seeking to settle in South Korea.

The 47-year-old man surnamed Kim, believed to be a former agent of the North's military intelligence unit, was charged with entering South Korea in June last year on a mission of pro-North espionage, according to the prosecutors.

Kim crossed Tumen River near the northern border of North Korea and traveled to Thailand with 14 other North Korean defectors through a broker, before arriving in the South, they said.

Questioning by prosecutors revealed that Kim had accepted the spy agency's suggestion to defect to the South for espionage activities while he was serving a 99-month disciplinary term for drug smuggling and human trafficking, which he committed during his previous work at the agency. 

Kim is not believed to have begun any anti-state activities in the South as the North Korean spy agency withheld a specific mission for him out of fear his identity would be discovered, prosecutors added.

The two Koreas are still technically at war since their 1950-53 Korean War ended in a truce, not a peace treaty. (Yonhap News)