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.

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

Asked specifically 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.

Thai Prime Minister Paetongtarn Shinawatra said when asked about the alleged deportations that the case had not been "discussed in detail".

She said any such action would have to be "based on the principles of the law... (and) human rights".

Thai authorities had repeatedly denied the existence of plans to repatriate the Uyghurs.

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 cross-border 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.

"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," she said.

- 'Irreparable harm' -

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

The United Nations indicated last month that it had urged Bangkok not to send them back to China after a group of UN experts said they would be "at risk of suffering irreparable harm".

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 a nearby airport.

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

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, adding that Beijing "opposes the use of Xinjiang-related issues as a pretext for interfering with normal law-enforcement cooperation among countries".