Chinese paramilitary police on patrol after a previous attack in Xinjiang. Photo: AP

At least 18 people died after ethnic Uighurs attacked police with knives and bombs at a traffic checkpoint in China's Xinjiang, Radio Free Asia reported on Wednesday.

The knife-and-bomb attack occurred on Monday in the city of Kashgar, the report said, as the Muslim minority group observes the holy month of Ramadan.

According to the Radio Free Asia report, the violence began when a police officer tried to stop a car that had gone through a checkpoint without stopping. The car backed up, running over the officer's leg and breaking it. Two unarmed traffic officers who came to help the injured officer were stabbed to death by two people who emerged from the car, Radio Free Asia reported, citing Turghun Memet, a local police officer.

By the time armed police officers reached the scene, more attackers had arrived. Fifteen assailants and three more officers were killed, Radio Free Asia quoted Memet as saying.

Details in the Radio Free Asia report were corroborated by at least three people in the neighborhood where the clash was said to have taken place. One, a police officer, who did not give his name because he was not permitted to talk to foreign news organizations, confirmed that such a clash had occurred and sent a photo of a document, which he said was a police notice, confirming the basic outlines of the Radio Free Asia report. It said that 15 attackers and two police officers had been killed and that the police had seized more than 100 firebombs, seven explosive devices and three large knives.

A bank employee and a person who answered the phone at a nearby hotel, both of whom declined to give their names, also confirmed a clash had occurred. "The incident wasn't very big. Everything is now back to normal," the person at the hotel said.

Repeated calls to the Xinjiang government news office were not answered. A person contacted at Tianshan Web, the region's official news portal, denied any attack had taken place, and wouldn't give a name.

Such incidents are frequently reported in overseas media but not confirmed by the Chinese government until days later, if ever.

Chinese media have not yet reported the incident, but if confirmed it would be the latest in a series of attacks in the region. Chinese President Xi Jinping has overseen a nationwide crackdown against alleged Uighur terrorists, that led to several shootouts, mass arrests and a stadium trial at which people were sentenced to death.

Xinjiang, a massive mainly desert region, has been hit by a series of terrorist attacks in recent years The Chinese government say that Islamic groups based across the border in Central Asia have been promoting religious extremism in Xinjiang.

Last year the Chinese government launched a crackdown on terrorist groups, following an attack in a crowded street market in the region's capital, Urumqi, in which 39 people were killed, and another at a railway station in the southwestern Chinese city of Kunming, in which 31 people were stabbed to death by people the police said were separatists from Xinjiang.

Last month police announced that they had broken up 181 violent terrorist groups in the past year, after targeting gangs, organizers and supporters, according to the official Xinhua News Agency