China must account for the whereabouts of ethnic Uighurs forcibly repatriated from Cambodia, a US-based rights group has said.
Human Rights Watch (HRW) said such groups had "disappeared into a black hole" on their return to China.
The Uighurs fled to Cambodia after mass ethnic riots in China in July. Beijing has referred to them as criminals.
In December, a group of 20 Uighurs were put on a plane to China despite opposition from the UN and US.
They said the group were likely to face persecution in China.
"Uighur asylum seekers sent back to China by Cambodia have disappeared into a black hole," said Sophie Richardson of HRW.
"There is no information about their whereabouts, no notification of any legal charges against them, and there are no guarantees they are safe from torture and ill-treatment."
HRW said a number of the group had given detailed accounts of past torture and persecution in China and that threats had been made against their families.
The organisation said China has a history of executing or imposing harsh sentences of Uighurs sent back from abroad and that there were unconfirmed reports some members of a group previously returned had been sentenced to death in western Xinjiang province.
Ms Richardson said the Chinese government must say where the group are being held and under what status as well as allowing the UN and family members to see them.
"Family members have the right to know what has happened to their loved ones," she said
"The Chinese government must treat all returnees humanely, ensure fair trials, and not persecute individuals for activities and speech that are protected under international law."
There has been no immediate comment from the Chinese foreign ministry.
The Uighurs fled Xinjiang after July's violent ethnic clashes in the provincial capital Urumqi which left at least 97 people dead.
Most of those killed in the unrest were majority Han Chinese, according to officials, and Urumqi's Han population had demanded swift justice.
At least 25 people have been 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.