As the Uighurs were put in a compound under constant guard in recent days, exile and rights groups grew increasingly nervous that Cambodia would give in to considerable pressure from Beijing, the Southeast Asian nation's largest foreign investor. The decision to expel them finally was announced on the eve of a visit from Chinese Vice President Xi Jinping.
Amnesty International warned the Uighurs could be tortured on their return to China, while analysts and exile groups said the deportation showed widespread problems with the refugee system at large.
The Uighurs, including two children, fled after ethnic rioting in July and slipped out of the country with the help of a secret missionary network in China. Their role in the rioting, China's worst communal violence in decades, remains unclear.
But China called them criminals, and Cambodia said it was deporting them because they had entered the country illegally. China's Foreign Ministry had no immediate comment Saturday night.
China has handed down at least 17 death sentences - mostly to Uighurs - over the summer violence between the ethnic minority and the majority Han Chinese. Exile groups say Uighurs have been rounded up in mass detentions since the rioting. Uighurs also complain the Chinese government has long restricted their rights, particularly clamping down on their practice of Islam.
Some countries have refused to send Uighurs - such as those released from U.S. detention at Guantanamo Bay in Cuba - back to China over concerns about retribution and abuse.
The U.S., the U.N. and several rights groups urged Cambodia not to deport the group, which, according to Interior Ministry spokesman Lt. Gen. Khieu Sopheak, left Phnom Penh International Airport on a special plane sent from China.
A spokeswoman for the United Nations High Commissioner for Refugees said it had not finished evaluating the Uighurs for refugee status and that expelling them was a "grave breach of international refugee law."
"I think we went to extraordinary lengths to prevent deportation. We had people prepared to try to physically prevent the deportation if it had taken place at the civilian side of the airport," said Kitty McKinsey, spokeswoman for the agency in Bangkok. The plane, however, left from the airport's military area.
The agency's high commissioner, Antonio Guterres, tried to speak to Prime Minister Hun Sen on the phone but was not successful, McKinsey added. She said the refugee agency is preparing a protest to the Cambodian government.
"It comes down to, it's up to states to provide protection," she said.
UNHCR has been slowly shifting some responsibilities for refugee evaluation to partner countries a development many rights groups find alarming. The agency has said that it works with Cambodian officials now to determine refugee status for asylum seekers and that eventually Cambodia would take over the job entirely.
"This deportation highlights the absolute ineffectiveness of the UNHCR," said Michael Horowitz, director of the International Religious Liberty project with the Washington-based Hudson Institute, who said he lobbied Cambodian officials not to deport the Uighurs.
Wang Lixiong, a China-based writer on Uighur and Tibetan issues, said the deportation reflected China's powerful influence in the region.
"When I learned the Uighurs landed in Cambodia, I was pessimistic because Cambodia is a small country that will not be able to stand against China's pressure," Wang said earlier.
Groups that had worked with the Uighurs had initially been more optimistic.
"Last Monday, the Cambodian government said to the UNHCR that they were committed to provide the conditions for a process of refugee status application for the Uighurs," said Taya Heng, legal officer of the Jesuit Refugee Service in Cambodia.
Ilshat Hassan, a U.S.-based director of the World Uyghur Congress, said he last spoke with the Uighurs two days ago, when they told him they had been put in a compound under Cambodian guard but that the U.N. refugee agency and the U.S. were monitoring the situation.
"We thought if it's the UNHCR it should be OK," he said. "The guys said the UNHCR officer said they will be under the protection of the U.N."
Some who worked with the Uighurs said U.S. State Department officials had directly urged Cambodia not to send them back to China.
"We are deeply disturbed by the reports that the Cambodian government might forcibly return this group of Uighurs without the benefit of a credible refugee status determination process," U.S. Embassy spokesman John Johnson in Phnom Penh said Saturday morning.
There was no immediate U.S. reaction after the expulsion.
After learning of the deportation, Ilshat said: "The UNHCR, the international world, the U.S., everybody who said something that could give us hope, they all failed."