(Updates with Thai immigration comment)

SEOUL, April 25 (Reuters) - Three young North Koreans who fled their communist country and were arrested later in Laos have been freed and are in South Korean custody at its embassy in Vientiane, a human rights group said on Wednesday.

The Lao communist government confirmed three children had been handed over to the South Korean mission on Tuesday, but insisted they were South Koreans who had been trafficked into the landlocked southeast Asian nation.

"The three kids have been trafficked," Lao Foreign Ministry spokesman Yong Chanhthalansy told Reuters by telephone.

"After investigations which took some time, the Lao government decided that although they entered illegally, they couldn't go on trial because they were under age," he said. The three -- two girls and a boy aged between 12 and 17 -- had fled North Korea for China and found their way to Laos, Peter Jung of the Seoul-based group Justice for North Korea said.

"They are now in the custody of the South Korean diplomatic mission," Jung said by telephone.

The South Korean Foreign Ministry, which usually refrains from commenting on North Korean refugees seeking asylum, declined to confirm the three were under its protection.

"We think the main priority is the security of these people," an official said. "Many countries are involved in the matter."

The three could become the first refugees to go directly to South Korea from Laos, one of North Korea's few allies and which is reluctant to offend its friends in Pyongyang.

Most North Korean refugees cross the border into China and then seek asylum -- via a third country -- in South Korea, where they are almost always granted citizenship.

Jung also said more than 400 North Korean refugees, 314 of them women, in an immigration detention centre in Thailand began a hunger strike on Tuesday to protest against a delay by the South Korean government in giving them permanent refuge.

"They are in deplorable conditions. Many of the women are ill," Jung said.

Thai immigration police said that was untrue.

"Nothing's happening," said Lieutenant Colonel Prawit Sirithorn, supervisor of the Bangkok Immigration Detention Centre. "Some of them might have been stressed out because of the heat."

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