22 countries, including Japan, are urging China to close mass detention centers holding members of its Muslim minority. They issued a joint letter to the UN Human Rights Council earlier this month.
International human rights groups say more than a million Uighurs are being detained in government camps for forced ideological education.
Beijing says it's fighting terrorism by offering vocational training to people influenced by extremist thinking.
In the letter, posted on the Human Rights Watch website, the countries say there are "credible reports" of arbitrary detention and widespread surveillance. They are demanding that China allow independent international observers into the Xinjiang region, where the camps are located.
China has yet to issue an official response. But Reuters, citing an anonymous diplomat, has reported that China is preparing a letter.
The joint statement comes as the Xinjiang region marks the tenth year since the eruption of ethnic clashes that killed about 200 people.