A prominent activist from Kazakhstan who is campaigning for the release of ethnic Kazakhs in China says he was forced to sign blank documents under house arrest.

Serikzhan Bilash, head of the advocacy group Atajurt, was accused of "inciting ethnic hatred" and placed under house arrest in the Kazakh capital of Astana earlier this week.

The charges against him have not been officially announced.

Bilash said in an audio message relayed by his lawyer that unknown officials visited his place on Wednesday and pressured him to sign blank documents.

The detention of possibly over a million Uighurs, Kazakhs and other ethnic minorities in Chinese internment camps has raised acute concerns in Kazakhstan which heavily relies on trade with neighboring China. Bilash's group has been actively supporting relatives of those detained.