"In large part, these have not been previously identified, so we could be talking about far greater numbers" of people detained, said Kyle Olbert, the director of operations for the movement.
"If anything, we are concerned that there may be more facilities that we have not been able to identify," he told a news conference 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.
Rights advocates have generally estimated that China is detaining more than one million Uighurs and members of other predominantly Muslim Turkic ethnicities.
But Randall Schriver, the top Pentagon official for Asia, said in May the figure was "likely closer to three million citizens" - an extraordinary number in a region of some 20 million people.
Mr Olbert said that archive imagery from alleged campsites showed consistent patterns - steel and concrete construction over the past four years along with security perimeters.
He said that 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.
China's foreign ministry said the allegations were "baseless."
"East Turkestan organizations outside China have long conducted activities harming China's national security," foreign ministry spokesman Geng Shuang said Wednesday at a regular press briefing.
He said it was "clear for all to see" that China's policies in Xinjiang had promoted "ethnic unity and social harmony" in recent years.