French authorities have opened an investigation into an anonymous threat against Chinese national carrier Air China, a spokesman for the Paris prosecutor's office said on Tuesday.

Air China's Paris office received a letter, written in English, threatening reprisals if Muslim prisoners in China were not released.

Suspected Muslim separatists killed 16 police officers in a homemade bomb attack in the northwestern region of Xinjiang last week, days before the opening ceremony of the Olympic Games.

In other attacks in Xinjiang, 11 people were killed in a series of supermarket bombings on Sunday and three security officers were stabbed to death and another wounded on Tuesday.

China says militants seeking an independent "East Turkestan" homeland for Muslim Uighurs in the region are among the top security threats to the Olympics.