The group has been held for up to three months at a detention centre in Bangkok, where they face harsh living conditions, activists in Seoul said.

"They are angry at extended delays in bringing them to the South," Lee Ho-Taeg, from the International Campaign to Block the Repatriation of North Korean Refugees, told AFP in Seoul.

He said some 400 refugees had been on hunger strike since late Tuesday, adding there were unconfirmed reports that the South Korean government had refused to grant them air tickets.

"About 300 women are held in the facility, which is barely enough for 100 people. There is only one toilet and more than 300 women have to share it," he said.

Thai officials declined to comment on the case.

"They are on a hunger strike to protest the delay of their departure from Thailand," UNHCR's deputy regional representative Giuseppe de Vincentis said.

"We have been trying to clarify this with the Thai government," but so far have received no answer, he added.

South Korean Foreign Minister Song Min-Soon confirmed the hunger strike without giving a number, and said Seoul had been in negotiations with Bangkok to "resolve the issue smoothly."

Tens of thousands of North Koreans, fleeing hunger or repression in the hardline communist state, have travelled across the border to China in recent years.

China has an agreement with its ally North Korea to repatriate them as economic migrants, a policy strongly criticised by refugee aid groups. The refugees often travel on to third countries in hopes of winning eventual resettlement in South Korea.

Thailand is an increasingly popular destination for the refugees, with the number of North Koreans arrested for illegal entry soaring to 400 in 2006 from 50 the year before.

But South Korean diplomats in the past have been accused by refugee groups of being unhelpful to the refugees, in order to avoid provoking Pyongyang.

Many are helped by rights organisations linked with Protestant churches in South Korea.

Lee said that in a separate case, three teenage defectors detained in Laos, who had feared possible repatriation to their homeland and harsh punishment, have been transferred to the South Korean embassy in the Lao capital Vientiane.

They have been identified as Choi Hyang, 13, and her 12-year-old brother Choi Hyok, who lost their mother in 1999 and came to China in 2002.

The third one is 17-year-old Choi Hyang-Mi, who arrived in China in 2001 with her mother but was parted from her after the mother married a Chinese man, Lee's group said.

The children were taken early Wednesday to Bangkok, where they were expected to connect to a flight to Seoul, said Hiroshi Kato of the Japan-based nonprofit group Life Funds for North Korean Refugees.

"The children have said they wanted to go to the United States, but we don't know if their wishes will come true," Kato said.

The number of northerners who had made it to South Korea since the end of the 1950-53 Korean war topped the 10,000 mark in February.

Estimates of the number hiding out in northeast China range from tens of thousands to hundreds of thousands.

Human Rights Watch last month said North Korea has toughened its punishments for people caught trying to flee -- including longer prison terms during which they face beatings and starvation.

Agence France Presse