The Vienna-based International Helsinki Federation for Human Rights said Yusuf Kadir Tohti and Abdulkadir Sidik went missing in Kazakhstan after being released from a pretrial detention center, where they had been held on accusation of possessing false passports and information of "an extremist character."
"Allegedly, they were secretly deported to China in violation of the non-refoulement principle," the group said in a statement. "At present no information is available as to their whereabouts, and they might have been secretly returned to China."
The two men's extradition have reportedly been requested by China, the group said.
A police officer in the Kazakh capital Almaty, who did not give his name because he was not authorized to speak to the media, said Friday that the two Uighurs were detained and then "voluntarily" bought air tickets and left for China last week.
The watchdog said there had been other cases of Uighurs disappearing in Kazakhstan, the AP reports.
China has long claimed that ethnic Uighurs militants are leading a violent Islamic separatist movement in northwestern China, and has repeatedly requested extradition of alleged Uighur separatists from neighboring ex-Soviet nations.
The Uighurs are Turkic-speaking Muslims with a language and culture distinct from the majority Chinese.
The FSB of Russia has distributed the footage of video surveillance over suspects, who allegedly worked to orchestrate a military coup in Belarus and planned to assassinate its president, Alexander Lukashenko