BEIJING (Reuters) - At least five police officers have been stabbed to death by separatists in a knife attack at a coal mine in China's violence-prone far western region of Xinjiang, U.S.-based Radio Free Asia reported on Wednesday.

The incident happened last Friday at the Sogan colliery in Aksu, the report said, citing police from the area. Dozens were also injured, it said.

The attack was "a long-planned, well-prepared, large-scale attack by separatists against police officers and mine owners at a coal field in our county", according to a government notice cited by Radio Free Asia.

Repeated calls to the Xinjiang government and public security departments were not answered. Such incidents are frequently reported in overseas media but not confirmed by the Chinese government until days later, if ever.

In June, Radio Free Asia said at least 18 people died when ethnic Uighurs attacked police with knives and bombs at a traffic checkpoint in the old Silk Road city of Kashgar.

The government has never confirmed that incident, although diplomats have told Reuters they believe a serious attack did take place.

Tensions between Muslim Uighurs that call the region home and the majority Han Chinese have resulted in bloodshed in recent years. Hundreds have been killed in violence across the region, blamed by Beijing on Islamist militants.

Exiled Uighur groups and human rights activists say repressive government policies in Xinjiang, including controls on Islam and Uighur culture, have provoked unrest, a claim that Beijing denies.