The US has expressed deep concern about the fate of 20 Uighur asylum seekers deported from Cambodia back to China.
A statement by the US embassy in Phnom Penh came a day after the Uighurs were put on a plane to China, despite pleas from the UN refugee agency.
The agency condemned the expulsions, saying Cambodia had committed a grave breach of international refugee law.
The Uighurs fled to Cambodia after mass ethnic riots in China in July. Beijing has referred to the group as criminals.
Human rights organisations have warned that the asylum seekers are likely to face persecution on return to China.
"The United States is deeply concerned about the welfare of these individuals, who had sought protection under international law," the US embassy in the Cambodian capital Phnom Penh said in Saturday's statement.
"We are also deeply disturbed that the Cambodian government decided to forcibly remove the group without appropriate participation by the office of the UN High Commissioner for Refugees."
The embassy also urged China to "uphold international norms and to ensure transparency, due process and proper treatment of persons in its territory".
Friday's expulsions followed intense pressure by China and came just two days before a visit to Cambodia by Chinese Vice-President Xi Jingping.
There has been no immediate comment from the Chinese foreign ministry.
At least 197 people died as violence flared in July, amid protests by Uighurs in the city of Urumqi, in Xinjiang region.
Shops were smashed and vehicles set alight while passers-by were set upon by Uighur rioters in the city, whose population is mostly from China's dominant Han group.
Groups of Han later went looking for revenge as police struggled to restore order.
Most of those killed in the unrest were Han, according to officials, and Urumqi's Han population had demanded swift justice.
Twelve people were sentenced to death after the riots.
Tensions between the mainly-Muslim Uighurs of Xinjiang and Han have been growing in recent years. Millions of Han have moved to the region in recent decades.
Many Uighurs want more autonomy and rights for their culture and religion than is allowed by Beijing's strict rule.