At least 24 people have been killed after a bus overturned in the country's restive western Xinjiang region, China's official news agency said.

The bus carrying students and parents was traveling Tuesday afternoon on a highway in a mountainous area of western Xinjiang, the official Xinhua News Agency said.

Most of the dead were students who were on their way to have health exams necessary for admission to high schools in China's more developed east, it said.

The cause is under investigation, Xinhua said.