Armed police in China's ethnically divided Xinjiang Thursday blocked the road to the site of riots that killed 27 people a day earlier in the region's deadliest violence in years, which state media called a "terrorist incident".

It was the deadliest spasm of violence to hit the troubled western region since 2009, when riots killed hundreds. Xinjiang, an area about twice the size of Turkey, is home to around 10 million members of the mostly Muslim Uighur ethnic minority.

Officers stationed 40 kilometres (25 miles) outside Turpan city's Lukqun township -- where the violence reportedly occurred -- checked car passengers' IDs and barred AFP journalists from entering, citing safety concerns.

A visitor in Turpan -- which lies about 250 kilometres from the regional capital Urumqi -- told AFP he saw another roadblock with armed officers and about 20 police vehicles.

A resident surnamed Chai said a helicopter arrived on the scene along with many police and soldiers.

The Xinhua state news agency said Wednesday that "knife-wielding mobs" attacked police stations and other locations, and nine police or security guards and eight civilians were killed before police opened fire.

The reason for the violence was not immediately clear, but state-run media on Thursday called it a "terrorist incident".

Chinese authorities have often blamed clashes in the region on "terrorists".

"Violent elements in Xinjiang are coming under the increasing influence of foreign terrorist forces," the Global Times, which has links to China's ruling Communist Party, said in an editorial.

Uighur rights groups have dismissed the claims of terrorism, citing economic inequality and religious repression by Chinese authorities as causes of sporadic unrest.

The World Uyghur Congress, a group run by exiled Uighurs, said the incident was "evidence of China's failed policies towards Uyghurs", in a statement, adding: "This incident has occurred around the arrival of Ramadan, which is severely repressed each year."

The group said "an information blackout and security crackdown" in the area called into question state media's account of the clashes.

China denies claims of repression, and insists its citizens have religious freedom. It has pumped investment into the region, which holds massive reserves of coal and gas, in an attempt to boost development and growth.

A handful of residents in Turpan interviewed by AFP on Thursday said they had heard about the incident but declined to elaborate.

Life in the city went on as normal as residents napped on the side of the road near mud-brick homes and grape fields.

According to official figures, 46 percent of Xinjiang's population are Uighur, while another 39 percent are Han Chinese, after millions from the majority group moved there in recent decades to find work.

The Han settlement drive has caused friction with the existing community.

Similar tensions have arisen in Tibet, which neighbours Xinjiang to the south.

Xinjiang saw its worst ethnic violence in years in July 2009 when riots involving Uighurs and Han settlers in Urumqi left around 200 people dead.

Chinese authorities closely restrict information about unrest in Xinjiang, blocking access across the region for several months after the violence in 2009.