Activists call it the 'underground railroad', harking back to the secret trails followed by African-American slaves fleeing to the free north in the 19th century. For North Koreans seeking a new life outside their impoverished homeland, it can be a road to freedom or a short-cut to arrest and deportation back into the hands of an unforgiving regime.
It's a journey that increasingly ends in Thailand, a key transit point for asylum seekers. After a long and often dangerous journey across China's vast hinterland and over its southern borders, North Koreans link up with Christian missionaries who guide them to Bangkok to await processing and resettlement. The next stop is normally a flight to Seoul, where more than 8,000 North Koreans have sought refuge in the past decade.
But the human traffic from North Korea is putting a strain on Thailand's policy of quiet co-operation with South Korea on the resettlement of defectors. Thai officials worry that their country is becoming a magnet for North Koreans at a time when it's still grappling with refugee outflows from neighbouring countries. About 150,000 Karen and other ethnic minorities from Myanmar are housed in squalid border camps, while ethnic Hmong from Laos continue to flee to Thailand, to the frustration of local authorities near the Laos-Thailand border.
Last month Thai police arrested 175 North Koreans who were staying at a rented house in Bangkok, the largest such group arrested in Thailand. That high-profile raid, which was filmed by Thai TV networks and flashed to South Korea, brought the number of North Koreans detained so far this year in Thailand to more than 400. Last year, the number arrested for the entire year was 80.
Activists working with refugees say the flow is unlikely to stop, as North Koreans already living illegally in China are looking for a safe haven. Some activists estimate that up to 300,000 are in northeast China, which shares a land border with its communist ally. Unable to settle legally in China, which considers them as economic migrants, more North Koreans are taking to the underground railroad.
'They come to Thailand because it's one of only a few countries where they can seek asylum ... Thailand is probably the best country to go to right now,' said Chun Ki-won, a South Korean missionary who was jailed in China in 2001 for his work. He estimates between 150 and 200 more North Koreans are hiding in Thailand, awaiting resettlement.