Turkey has called on China to close its detention camps following the reported death of a renowned musician from the ethnic Uighur minority.
Abdurehim Heyit is thought to have been serving an eight-year sentence in the Xinjiang region, where a million Uighurs are reportedly being detained.
A statement from Turkey's foreign ministry said they were being subjected to "torture" in "concentration camps".
China says the facilities are re-education camps.
The Uighurs are a Muslim Turkic-speaking minority based in the north-west Xinjiang region of China, which has come under intense surveillance by Chinese authorities.
Beijing claims that the detention camps in Xinjiang are "vocational education centres" designed to help rid the region of terrorism.
Who are the Uighurs?
The Uighurs make up about 45% of the population in Xinjiang.
They see themselves as culturally and ethnically close to Central Asian nations, and their language is similar to Turkish.
In recent decades, large numbers of Han Chinese (China's ethnic majority) have migrated to Xinjiang, and the Uighurs feel their culture and livelihoods are under threat.
Xinjiang is officially designated as an autonomous region within China, like Tibet to its south.