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.

Six more defectors and a South Korean activist were arrested in northern Thailand last week, according to the South Korean news agency Yonhap. The detainees included a six-month-old baby and five young women, it reported, citing an activist source.

Their route is one that's familiar to Lee Dong-soo, a 22-year-old factory worker from North Korea who left his home in June and set out to reach Thailand. His two sisters had left and found their way to South Korea, and Mr Lee - not his real name - wanted to join them. From relatives living across the border in China, he heard about the backdoor route to Thailand, and a chance at resettlement in South Korea. So he borrowed some money and set off, travelling by bus across China with a nephew and several other North Koreans.

Getting stopped in China was a dangerous proposition. If he was deported back to North Korea, authorities there could prosecute him for leaving without permission - a serious crime.

Mr Lee said he was already at risk because his sisters had defected, though only the family knew that. 'I was afraid that someone would find out [about the defections]. Then the government could send me to a detention centre,' he said.

He crossed China without incident, sneaked across the border into Laos and eventually entered northern Thailand by boat. His group met a South Korean activist who led them to a safe house in Bangkok, a two-storey corner mansion in a gated community close to the South Korean embassy. He was told to wait until his paperwork was ready for resettlement.

On August 22, his luck ran out. Acting on complaints from neighbours, who claimed that the North Koreans were loud and disruptive, police raided the house and detained the 175 people living there. Two days later, Mr Lee was among 136 North Koreans sentenced to 30 days in prison after being found guilty of illegally entering Thailand. Another 18 were cleared to leave for South Korea, which had already granted them citizenship.

Last week, the UN High Commissioner for Refugees, which recognises all North Koreans fleeing to Thailand as 'Peoples of Concern', began registering those sentenced to prison.

'We're trying to ascertain what they want to do, if they want to go to a third country,' said Kitty McKinsey, a spokeswoman for UNHCR. 'In the past this has usually been South Korea.'

But for Mr Lee and his fellow detainees, there's another option on the table.

Under the 2004 North Korean Human Rights Act passed by Congress, the US provides fast-track access for asylum seekers, though only a handful have taken up the offer. Lee, who set out to reach South Korea, says he's weighing up his options.

'I think we can enjoy freedom in the US,' he said. 'I hear it's a more developed country.'

Mr Chun, the missionary activist, says that 30 more applicants who fled to Southeast Asia before the latest arrests in Bangkok are waiting for approval. He recently travelled to the US to talk to Korean church leaders about helping new arrivals to assimilate.

US officials say that while they anticipate more applicants, South Korea will likely remain the top choice, given kinship and proximity.

'It's a natural tendency to go home, or as close to home as possible, and for most North Koreans, South Korea would seem the logical destination where they'd feel at home,' US Assistant Secretary of State Ellen Sauerbrey said in Bangkok last week.

But critics are sceptical of the US determination to reach out to refugees who aren't in safe havens like Thailand. 'In order for the act to be really effective, US diplomatic missions in a number of countries must be flexible and creative in the way that they offer sanctuary to refugees,' said Tim Peters, a US evangelical pastor who runs Helping Hands Korea, a charity based in Seoul.

Among those countries is China, perhaps the most dangerous way-station along the underground railroad. In recent years Beijing has tightened security at foreign missions in China to stop North Koreans claiming asylum after a spate of high-profile cases, including the scaling of embassy walls by defectors. Many missions are now virtually impregnable.

China has also rebuffed requests by the UNHCR to expand its operations to the northeast border. US and UN officials say they have repeatedly urged China to stop the expulsion of asylum seekers, noting its obligations under the 1951 UN Refugee Convention.

'We are aggressively encouraging all governments in the region to provide opportunities for all North Koreans who reach their destination to allow them to move on to resettle in third countries,' Ms Sauerbrey said.

While Mr Lee covered the distance quickly, and without getting caught, other refugees struggle to reach Thailand. Many live illegally in China for several years before deciding to flee, often guided by activists from South Korea.

Refugees say Chinese traffickers profit from the trade. They typically demand an upfront payment with the promise of a money transfer if refugees reach South Korea - which gives US$10,500 as a one-off payment to new citizens. Some send money back to relatives in North Korea or China, effectively funding the next migratory wave.

After her mother fell sick, Jun Jae-youn left North Korea in 1998 and later married a Chinese man. They had a daughter, and she tried to settle into her new life in northeast China. 'I could live my daily life. But I wasn't a citizen, I was illegal. I always felt that burden.'

After she heard that some of her relatives had defected to South Korea, Ms Jun - a pseudonym - decided to follow them, even though it meant leaving her husband and child. Earlier this year, together with four other North Koreans, she crossed into northern Thailand, and ran into a police checkpoint. The group was deported across the border to Myanmar, but sneaked back again into Thailand.

Finally, after several run-ins with Thai police, they reached the safe house in Bangkok, and were told by activists to lie low until passage to South Korea could be arranged. A week later, the house was raided, and Ms Jun found herself in a Thai prison with her housemates. Asked if she wants to go to the US or South Korea, Ms Jun knits her brow. 'I've not decided. I just want to go somewhere safe.'