(CNN) United Nations Secretary-General Antonio Guterres has reminded China that its treatment of Uyghurs is still under close watch. Human rights must be respected even when fighting terrorism, he told Chinese authorities during a visit to last week's billion-dollar Belt and Road Forum in Beijing, according to a UN spokesman.

China has cracked down on its population of Muslim Uyghurs, who are concentrated in the country's far western Xinjiang region. Last year, a US State Department official estimated that at least 800,000 and possibly up to two million people may have been detained in huge "re-education centers." In February, Turkey's Foreign Ministry spokesman Hami Aksoy said the people detained in the camps were subject to "torture and political brainwashing" and called the camps a "great shame for humanity."

China has defended its use of what it calls vocational training centers, saying they are a necessary counterterror measure.

During Guterres' trip to Beijing last week, he met with China's President Xi Jinping. Asked by reporters on Monday whether Guterres had raised the issue of Uyghurs during that conversation, UN spokesman Stephane Dujarric replied that the UN chief had "discussed all relevant issues with Chinese authorities."

"He did just that, and that includes the situation in Xinjiang," he said.

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