The law, passed Thursday by the National People's Congress, formalises policies to promote Mandarin as the "national common language" in education, official business and public places.

Rights advocates warn the law could further marginalise minority groups such as the Uyghurs.

Turk is "concerned" by the law, his office said on X.

"This risks entrenching assimilationist policies in statute, restricting minority-language education, and limiting free practice of religion and culture.

"Its provisions could overly restrict freedoms of expression, belief and assembly and penalise peaceful exercise of minority rights generally.

"International human rights law requires states to protect identities of ethnic, linguistic and religious minorities."

China's government has been accused for decades of pursuing policies to force assimilation across the vast country into the Han majority.

The law aims to "strengthen cohesion" within China, which the legislation argues is undergoing unprecedented social change.

China officially recognises 55 official ethnic minorities within its borders that speak hundreds of languages and dialects.