Thailand on Thursday deported 40 Uyghur asylum seekers back to China, drawing a sharp rebuke from the United Nations’ refugee agency and activists who had long warned that the men would possibly face torture and long-term imprisonment upon their return.

Thailand’s police chief, Kittirat Panpetch, confirmed the move at a news briefing on Thursday, saying that Thailand had deported the Uyghurs at the request of the Chinese government. He said the matter had been handled by the country’s police and national security council.

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

China has used its power and influence to silence its critics abroad and pressure governments to repatriate citizens fleeing persecution. The group of Uyghurs, a persecuted Muslim minority in China, had been detained in Bangkok for over a decade. They were part of a wave of more than 300 people who fled China in 2014, hoping to use Thailand as a transit point to get to Turkey, which is home to a sizable Uyghur community. Last month, some of the detainees, who are all men, went on a hunger strike amid fears of being returned to China.

A photograph provided on condition of anonymity showed Uyghur detainees sitting in an immigration detention center in Bangkok, in February.Credit…Associated Press

The plight of the Uyghur detainees in Thailand has drawn scrutiny from many governments, including the United States. During his confirmation hearing for secretary of state last month, Marco Rubio said he would lobby Thailand not to send the Uyghurs back to China. And on Tuesday, U.S. Senate Foreign Relations Committee members Jim Risch and Jeanne Shaheen warned that any deportations would be “ill-advised.”

Rights activists who had been monitoring the situation began reporting in the early hours of Thursday that there were signs the Uyghurs were being prepared for deportation.

In Bangkok at around 2 a.m., a reporter witnessed six trucks that had their windows covered with black cloth leaving an immigrant detention center in downtown Bangkok where the detained Uyghurs had been held. Several police cars accompanied the trucks, cordoning off traffic around them.

A few hours later, an unscheduled China Southern Airlines flight took off from Bangkok to Kashgar in Xinjiang, the native homeland of Uyghurs, according to FlightRadar24, which tracks flights around the world. It landed just after 12 p.m. local time.

“All signs point to at least 40 of the men having been deported,” said Julie Millsap of No Business With Genocide, a Washington-based group that has been lobbying governments to free the Uyghurs.

In a statement, Human Rights Watch criticized the Thai authorities for having deported the men despite making public assurances earlier that they would not do so.

“Thailand’s transfer of Uyghur detainees to China constitutes a blatant violation of Thailand’s obligations under domestic and international laws,” said Elaine Pearson, Asia director at Human Rights Watch. “The men now face a high risk of torture, enforced disappearance, and long-term imprisonment in China.”

A Chinese foreign ministry spokesman did not directly answer a question about whether the Uyghurs had been deported, saying that 40 Chinese nationals who had illegally entered Thailand had been repatriated legally on Thursday. China’s official news agency, Xinhua, which carried a report on the return of the “illegal immigrants,” appeared to be deliberately vague about the deportees, providing no details regarding their identities or where in China they were from.

The Thai foreign ministry did not respond to repeated requests for comment. The police chief, Police General Kittirat, said that in its letter to the Thai authorities seeking the deportations, the Chinese government had pledged to take care of the Uyghurs.

Pirada Anuwech contributed reporting from Bangkok.