“In large part these have not been previously identified, so we could be talking about far greater numbers” of people detained, Kyle Olbert, the director of operations for the movement, said on Tuesday.

“If anything, we are concerned that there may be more facilities that we have not been able to identify,” he said in suburban Washington.

Anders Corr, an analyst who formerly worked in US intelligence and who advised the group, said that around 40 per cent of the sites had not been previously reported.

Pompeo says US ‘troubled’ by reports Uygur activists’ families harassed

Rights advocates have generally estimated that China is detaining more than one million Uygurs and members of other predominantly Muslim Turkic ethnicities.

But Randall Schriver, the top Pentagon official for Asia, said in May that the figure was “likely closer to three million citizens” – an extraordinary number in a region of some 20 million people.

Olbert said that archive imagery from alleged camp sites showed consistent patterns such as steel and concrete construction over the past four years along with security perimeters.

He said the group tried to verify the nature of each site with on-the-ground accounts but declined to give greater detail, citing the need to protect sources.

Olbert described China’s policy as “genocide by incarceration”, fearing that Uygurs would be held indefinitely.

“It’s like boiling a frog. If they were to kill 10,000 people a day, the world might take notice,” he said.

“But if they were just to keep everyone imprisoned and let them die off naturally, perhaps the world might not notice. I think that’s what China is banking on.”

China has justified its policy after first denying the camps, saying that it is providing vocational training and coaxing Muslims away from extremism. Hundreds died in 2009 riots in Xinjiang’s capital Urumqi that largely targeted Han Chinese.

Pompeo urges NBA to stand up to China, comparing Xinjiang to Orwell’s 1984

The United States has likened China’s treatment of Uygurs to Nazi Germany’s concentration camps, but an increasingly strong Beijing has faced limited criticism outside the West.

China last month secured a statement at the United Nations by a number of countries including Russia, Pakistan and Egypt – which have all faced criticism of their own records – that praised Beijing’s “remarkable achievements in the field of human rights”.

The movement said it had unsuccessfully asked the State Department for satellite data in hopes of improving its information sources.

US lawmakers have also spoken out increasingly on Xinjiang.