(Corrects to add dropped word 'how' from first paragraph)
* Xinjiang official declines to give figure for number detained
* U.S. says there is no evidence to support China's claims
* Rights group says there are no reports of large-scale releases
By Michael Martina
BEIJING, July 30 (Reuters) - Most people sent to mass detention centres in China's Xinjiang region have "returned to society", a senior official from the area said on Tuesday, but he declined to give an estimate of how many have been held in recent years.
A spokeswoman for the U.S. State Department said there was no evidence to support the assertion made by Xinjiang's vice chairman, Alken Tuniaz, and said Beijing should allow the U.N. High Commissioner for Human Rights unhindered access to assess the claim.
U.N. experts and activists say at least 1 million ethnic Uighurs, and members of other largely Muslim minority groups, have been detained in camps in the region of western China.
Beijing describes the camps as vocational training centres to help stamp out religious extremism and teach new work skills.
Tuniaz, asked at a briefing in Beijing for an account of how many people had been put in the facilities, said the number was "dynamic", and that most had "successfully achieved employment".
"Currently, most people who have received training have already returned to society, returned home," he said.
A transcript of the briefing emailed to reporters had been edited to read "most have already graduated", using the word for students who finish a course or graduate from high school.
"Individual countries and news media have ulterior motives, have inverted right and wrong, and slandered and smeared (China)" over the centres, he said.
China has not issued any detailed figures for how many people have been sent to the camps and authorities limit access for independent investigators.
Researchers have made estimates through various methods such as analysing government procurement documents and satellite imagery of the facilities.
Foreign journalists have reported personal accounts of some former internees, and photographed sprawling prison-like facilities surrounded by razor wire and watch towers.
As Western countries have mounted more strident criticism of the camps, China has not backed down on what it says is a highly successful de-radicalisation programme in a region that has been plagued with intermittent ethnic violence.
Officials have arranged highly choreographed visits for journalists and diplomats to some of the facilities, where the government says the rights of the "trainees" are fully guaranteed.
It has also suggested that fewer people would be sent through the centres over time.
The government rejects any suggestion that it abuses religious and human rights.
U.S. Secretary of State Mike Pompeo this month called China's treatment of Uighurs and other minorities in Xinjiang the "stain of the century", and the Trump administration has been weighing sanctions against Chinese officials over their policies there.
"We are unable to verify the vague claims made by senior Chinese officials regarding the release of those arbitrarily detained", the State Department spokeswoman said.
"In order to assess these claims, the Chinese government should allow the U.N. High Commissioner for Human Rights unhindered and unmonitored access to all camps and all detainees - which has yet to occur".
She said the Chinese government should also allow targeted minorities to freely travel away from Xinjiang and China to promote transparency.
Nicholas Bequelin, Amnesty International's regional director for East and Southeast Asia, also said China's claims were not credible. He said his group had received no reports of large-scale releases and detainees were still being held incommunicado.
"China is making deceptive and unverifiable statements in a vain attempt to allay worldwide concern for the mass detentions of Uyghurs and members of other ethnic minorities in Xinjiang," Bequelin said in a statement.
(Reporting by Michael Martina in BEIJING and David Brunnstrom in WASHINGTON Editing by Robert Birsel and G Crosse)
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