Beijing is accused of committing human rights abuses in its northwestern Xinjiang region, including the incarceration of around one million Uyghurs and other Muslim minorities. It denies the allegations.

Rights groups had warned in recent weeks that Bangkok was preparing to deport a group of 48 Uyghurs being held in immigration centres around Thailand.

They said the Uyghurs fled China more than a decade ago and lived in constant fear of being sent back.

Thai national police chief Kitrat Phanphet confirmed the deportations on Thursday, saying Beijing had requested the return of 40 Uyghurs.

"The Chinese government wrote to the Thai government expressing sincerity and their intention to take care of Uyghurs, promising in the letter that they will be looked after, and guaranteed their accommodation and their safety," Kitrat told reporters.

"We follow the procedure of international principle and laws," he told reporters, saying the handover went smoothly and the Uyghurs did not resist.

Thailand forcibly deported 109 Uyghurs to China in 2015, drawing a stern rebuke from Washington and the UN, but had repeatedly denied the existence of plans for a new round of deportations.

The UN indicated last month that it had urged Bangkok not to go ahead after a group of its experts said the deportees would be "at risk of suffering irreparable harm".

The UN High Commissioner for Refugees said on Thursday it "deeply regrets the deportation".

"This is a clear violation of the principle of non-refoulement and the Royal Thai Government's obligations under international law," the UNHCR said in a statement.

- 'Concrete measure of cooperation' -

China's public security ministry said on Thursday 40 Chinese "illegal migrants" had been deported from Thailand "in accordance with... international law".

Asked whether the group included Uyghur detainees, Beijing's foreign ministry said only that they had "Chinese nationality".

"The repatriations... were a concrete measure of cooperation between (China and Thailand) in combating cross-border crimes," ministry spokesman Lin Jian told a regular news briefing.

"The legitimate rights and interests of the persons concerned were fully protected," Lin said. He declined to give further details.

Chinese public security officials said in a statement the deportees "were lured by criminal organisations into illegally leaving the country and remaining in Thailand".

"They and their families have suffered great harm, and their relatives have repeatedly requested that the Chinese government provide assistance so that they can return to the embrace of the motherland," the unidentified officials said.

Beijing and Bangkok are working closely to return thousands of Chinese workers from online scam compounds, with Thailand acting as a bridgehead for returns.

However, most of those compounds are based in Myanmar, with few if any workers staying in Thailand for prolonged periods.

- 'Irreparable harm' -

The group of 48 Uyghurs, arrested in 2013 and 2014, were being held in immigration centres across Thailand.

Thai authorities had allegedly held the group in de facto detention for more than a decade with no access to lawyers or family members.

Campaigners warned on Wednesday that detention facility officials were preparing to transfer the Uyghurs to Don Mueang airport, Bangkok's secondary international terminal.

Flight tracking websites later showed a Chinese commercial plane departing Bangkok for Kashgar in Xinjiang.

Rights groups and Uyghurs overseas allege that China has detained more than a million Muslims, mostly Uyghurs, in a network of facilities in Xinjiang that are rife with violence, torture, forced labour, political indoctrination and other abuses.

The UN has said Beijing's actions may constitute "crimes against humanity" and the United States has branded them a "genocide".

China vehemently denies the accusations, saying its policies in Xinjiang have eradicated extremism and boosted development, and that the facilities were voluntarily attended training centres that closed years ago after attendees "graduated".

On Thursday, China's foreign ministry accused "some political forces" of spreading lies about Xinjiang.

Rights groups slammed the deportation, with Human Rights Watch calling it "a blatant violation of Thailand's obligations under domestic and international laws".

"The men now face a high risk of torture, enforced disappearance, and long-term imprisonment in China," its Asia director Elaine Pearson said.