The groups say the case throws a spotlight on Thailand's lack of readiness to care for undocumented children, whose numbers are rising, fuelled by growing instability and violence in Myanmar, after a military coup in 2021.

The children, aged between five and 16, have been denied access to parents and relatives since March 12, when government social workers took them from a shelter in the northern province of Chiang Rai, rights worker Nutchanart Boonkhong told Reuters.

"We saw how the government works and we are very concerned," said Nutchanart, who set up the foundation running the shelter.

She cited a similar incident last July when 126 children, mostly belonging to ethnic minorities, were sent to Myanmar by Thai authorities because they were undocumented, sparking an uproar by civil society groups.

"That group was deported, and it is unknown whether there was any follow-up on their welfare," she added.

At the time, officials said they were following rules on deportation after discovering numerous undocumented children at a public school in the central province of Ang Thong, far from the border.

But Varawut Silpa-archa, Thailand's minister for social development and human security, denied there was any deportation plan in the current case.

"All children in Thailand, regardless of their ethnicity, will be taken care of ... There is no policy to send them back," Varawut said.

The group was taken into custody over fears that a temple in the province of Lopburi was exploiting them to raise funds, he added.

Responding to the accusation, Nutchanart said the children had been spending their summer at the Lopburi temple and were not abused.

Thailand is home to hundreds of thousands of stateless people, whose families criss-crossed porous borders in the remote hills between southern China, Myanmar and Laos for generations before arriving in the country.

Many still have difficulties accessing services such as education.

"There are already 500,000 undocumented children who fled to Thailand from Myanmar and at least 1.5 million will follow," said former human rights commissioner Tuenjai Deetes.

"Thai government policy has not adjusted to this reality."

(Reporting by Panu Wongcha-um and Panarat Thepgumpanat; Editing by Clarence Fernandez)