Five school girls were injured in the blaze which broke outat the the two-storey building of Pithakkiart Witthaya School in Chiang Rai province late last night.
The girls who died in the fire were aged between five and 12 years old. Two of the injured were in a serious condition.
There were 38 schoolgirls staying at the boarding-house, most of whom were asleep when the fire broke and were unable to escape the blaze.
After the blaze broke out officials could only evacuate a few of the girls while the rest were trapped inside.
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News Channel 3 reported that most of the victims were burned beyond recognition.
Rescuers picked through the debris late last night and into the morning today.
Sawang Momdee, the Chiang Rai chief disaster mitigation officer, said disaster mitigation officials, rescuers and police combed through the boarding-house trying to locate the two missing girls, The Nation reported.
Firefighters took three hours to put out the fire, and pulled survivors and bodies from a window on the second floor of the wooden building.
Thailand, which is home to hill tribes who mainly live in the remote northern area bordering Laos and Myanmar, has poor health and safety standards and accidents are common across the country.