They fled North Korea in 2000 and had been hiding in China until last November, when they made a failed attempt to reach Thailand via Laos and obtain refugee status in hopes of eventually living in South Korea.

The young North Koreans believed Thailand could help them because it is the home country of Vitit Muntarbhorn, the UN high commissioner for human rights who has called for better treatment of North Korean defectors, according to the letter obtained by media outlets here.

Choe Hyok, a 12-year-old boy, his 14-year-old sister, Choe Hyang, and another girl, Choe Hyang-Mi, 17, were captured by Laotian border guards while crossing the Mekong River en route to Thailand.

They were given a three-month prison term for illegal entry into Laos. But they are still being detained.

Laotian authorities have said they would hand the children to North Korean diplomats if Laos isn't paid $1,000 for each detainee, according to human rights activists.

A Japan-based human rights group, which has tried to intervene, has rejected the demand, informing South Korea of the detainment.

In the letter, delivered through the human rights group, the children said: "We would rather kill ourselves than be taken to North Korea because we are set to be executed when taken back." They requested asylum in South Korea.

However, South Korean officials in Laos have neither visited the young people nor taken any action.

South Korea has maintained a low-key stance toward human rights abuse in North Korea for fear of creating friction with Pyongyang.

In the face of mounting criticism, the Seoul government established contact with Laotian officials.

"We have been talking with Laos for the release of the North Korean teenagers," a Foreign Ministry official said. "We are trying to prevent their forced repatriation to North Korea and allow their settlement in their favored destination, possibly South Korea," he said.

North Korean defectors wearing black hoods stage a rally to demand the release of North Korean teenagers held in Laos, in front of the Laos Embassy in Seoul on April 13. AP/Ahn Young-joon

A group of human rights activists have staged rallies in front of the Laotian embassy in Seoul to call for the release of the youngsters.

Most North Koreans seeking asylum in the South Korea do so via China. In the wake of harsh crackdown in China, however, more and more North Koreans have made their way to Southeast Asian countries.