Chinese police shot dead 12 people during unrest in the western Xinjiang region, the government said Sunday, in a rare admission security forces opened fire in the worst ethnic violence in decades.

Police shot and killed 12 "mobsters" during disturbances in the regional capital Urumqi on July 5, Xinhua news agency said in a report issued early Sunday that quoted the head of the Xinjiang regional government, Nur Bekri.

Some of Xinjiang's Uighur minority, a mainly Muslim, central Asian people, went on the rampage on July 5, assaulting members of China's dominant Han ethnic group in attacks that left at least 192 dead.

Uighurs say police sparked the rioting by shooting peaceful protesters who were demanding an investigation into a recent factory brawl in southern China that left at least two Uighur migrant workers dead.

However, Nur Bekri said police opened fire to prevent further bloodshed, according to Xinhua.

"The police showed as much restraint as possible during the unrest," he said, adding they had initially fired shots into the air but that had failed to disperse "extremely vicious" thugs.

Three of those shot died on the spot, with nine others dying after medical treatment failed, he said.

The report gave no details of the ethnicity of the deceased or those involved in the unrest, but authorities have already pinned the blame for the unrest on Uighurs, many of whom complain of decades of Chinese repression.

Thousands of Han Chinese armed themselves with clubs and other weapons and marched through Urumqi seeking vengeance on Uighurs in the days after the riots, but were mostly thwarted by a huge security force.

Xinhua also issued a report Sunday that it said supported the government's official line that the unrest was planned in advance.

Citing unnamed witnesses in Urumqi, it said rioters instigated violent acts in more than 50 locations throughout Urumqi, suggesting that weapons were stockpiled ahead of time.

"Knives became hot-selling products two or three days before the unrest," it said, citing local businesses.

It also said stones that were hurled at targets or used to bludgeon victims appeared to have been trucked into the city in large numbers beforehand and that rioters appeared to have knowledge of how to quickly ignite the fuel tanks of buses and other vehicles.

The report did not say who might have planned the attacks.

However, China has already said they were orchestrated by exiled US-based Uighur activist Rebiya Kadeer, who has denied the accusation.

The Xinhua reports made no mention of ethnic violence and said the unrest had initially started as a "student parade".

In the wake of the disturbances, Chinese troops and police imposed heavy security on parts of Urumqi, particularly neighbourhoods with large Uighur populations.