BEIJING — Thailand has sent about 100 ethnic Uighurs back to China, the country they fled, a Thai government spokesman said on Thursday. Thailand acted despite international warnings that the Uighurs could face harsh treatment in China and pleas that they be allowed to resettle elsewhere.

The Office of the United Nations High Commissioner for Refugees called the deportation of the Uighurs “a flagrant violation of international law.”

The Thai spokesman, Maj. Gen. Weerachon Sukhonthapatipak, said at a news conference in Bangkok that the Uighurs who were sent back Thursday had been detained in Thailand since last year and that the government had thoroughly established their country of origin. He said another 50 Uighurs whose nationality was unclear remained in Thailand. About 170 Uighurs were previously found to be Turkish citizens and were sent to Turkey, despite lobbying from China, the general said.

Uighurs are a mainly Muslim ethnic minority who speak a language related to Turkish and are concentrated in Xinjiang, a vast region in far western China. They have chafed under Chinese rule, and violence between Uighurs and Han, the dominant ethnic group in China, has been rising in recent years. Han civilians and government offices have been attacked, and Chinese security forces have clamped down on Uighur towns. Uighurs also complain about Han migration to the region and increasingly severe policies aimed at undermining the practice of Islam.