Thailand has repatriated around 100 ethnic Uighur migrants to China, where rights groups warn they could face persecution.
The Turkic-speaking minority group has long complained of official discrimination in China's Xinjiang region. Waves of Uighurs have fled the area.
Many have taken a route through Southeast Asian countries, including Thailand, where they are sometimes detained as illegal immigrants.
Thai government spokesman Maj. Gen. Verachon Sukhonthapatipak said Thursday the Uighur migrants in question had been detained for over a year.
The spokesman said the migrants were sent back to China after it was determined they were Chinese. About 170 others were found to be Turkish and sent to Turkey.
He said Thailand received assurances from Beijing that the migrants would not be harmed, but that failed to satisfy many human rights groups.
Sunai Phasuk of Human Rights Watch called the move "appalling," saying on Twitter that the Uighurs "could face torture and other serious abuses."
The United Nations refugee agency also said it was "alarmed at today’s deportation of some 100 persons of Turkic origin by Thailand."
China had put intense pressure on Thailand to return the Uighurs, and expressed displeasure earlier this week when the others were sent to Turkey.
Beijing has launched an intense crackdown on the mostly Muslim group in Xinjiang province, where there has been a series of militant attacks in recent years.
Exiled Uighur rights groups say the terror attacks are acts of desperation caused by government policies that favor members of China's Han majority group.
China denies mistreating the Uighurs, saying it is only interested in bringing economic development to the energy-rich region.