The Thai government has suffered intense criticism from around the world for its decision to hand over at least 40 Uyghurs, who were flown by special plane to China's northwestern Xinjiang region on Thursday.

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

The bulletin from the US embassy in Bangkok noted that the last deportation of Uyghurs from Thailand in 2015 was followed by a bomb attack on a shrine in the Thai capital that killed at least 20 people.

US citizens should "exercise increased caution and vigilance, especially in crowded locations frequented by tourists due to the potential for increased collateral risk", the warning said.

"Similar deportations have prompted violent retaliatory attacks in the past," it added.

The Uyghurs had spent years languishing in Thai detention facilities after fleeing China more than a decade ago.

Thailand, which forcibly deported 109 Uyghurs to China in 2015, had repeatedly denied the existence of plans for a new round of deportations.

But rights groups warned in recent weeks that Bangkok was preparing to deport a group of 48, and early on Thursday they were put onto a flight to Kashgar.

The exact number returned is not clear. China said 40, the US embassy statement said 45 and rights groups have given a figure of 48.

- Condemnation -

US Secretary of State Marco Rubio slammed Thailand -- a longstanding ally -- on Thursday "in the strongest possible terms".

Germany, Britain, the United Nations and the European Union all joined the international chorus of disapproval.

But Thailand has defended its decision, saying China assured that the Uyghurs would be well treated.

Prime Minister Paetongtarn Shinawatra, who recently held talks in Beijing with President Xi Jinping, insisted the Uyghurs' return was not part of any trade bargain.

"There have been no trade talks related to the repatriation of Uyghurs at all. There are none. These issues are unrelated," she told reporters on Friday.

"I had confidence that once they were returned, China would take good care of this group of people."

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.

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".