In one of the deadliest knife attacks in China's restive Xinjiang province, as many as 40 people, including five police officials, have reportedly been killed by militants.

Suspected separatists attacked the owner and security guards at a coal mine in Sogan Colliery in Aksu prefecture last Friday, and then attacked police officials who arrived at the scene, leaving at least 40 people dead, Radio Free Asia has reported. 

The knife 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", the police said. 

"At least 40 people were killed or injured, including police officers, security guards, mine owners and managers, and attackers," Jamal Eysa, the chief of state security police, told RFA. 

The attackers also took control of the dynamite at the colliery, but it is not clear if they used the explosives. 

"That is why they were able to do such severe damage to our police team and to the Han businessman and factory owners," Eysa said. 

Some of the attackers are believed to be on the run after the deadly attack, the report said. 

China has seen a spate of knife attacks in recent years, with many carried out by Uighur Muslims as a response to alleged religious repression by the Chinese government. 

In June, 18 people were killed when Uighur Muslims attacked a police station in Xinjiang. 

Last year, a knife attack at a train station in Kunming city left nearly 30 dead.

China has vowed to crack down on terrorism, jailing scores of Uighurs and executing militants. 

Last month, China jailed 45 people from Xinjiang on terror charges.