BEIJING (AFP/REUTERS) - An unknown number of people have been killed and injured in an attack on an open-air market in Urumqi, capital of the restive Chinese region of Xinjiang which is home to mostly Muslim Uighurs, the official Xinhua news agency reported on Thursday.

Two off-road vehicles ploughed into people at the market and explosives were thrown out of the cars, one of which exploded, Xinhua said citing witnesses.

Flames and heavy smoke were seen nearby while the area had been cordoned off after the blast, which took place at around 8am local time (0000 GMT), Xinhua added.

China has blamed a series of knife and bomb attacks in recent months on separatist militants from

Xinjiang, the traditional home of the ethnic Uighur people.

Xinjiang has been plagued by violence for years, but rights activists and exile groups say the government's own heavy handed policies in the region have sowed the seeds of unrest.

Photos posted on social media purportedly of the blast, but unverified by Reuters, showed a column of smoke and chaos at a fruit and vegetable market, with injured people lying on the road. Those injured were rushed to hospital, Xinhua said.

A witness at the market told Xinhua he heard a dozen "big bangs" during the incident when Chinese morning markets, which usually sell fresh groceries, are commonly crowded with shoppers.

"There were multiple strong explosions in the morning market at the Cultural Palace in Urumqi," wrote one Weibo poster who said he was less than 100 metres (yards) from the scene.

"I saw flames and heavy smoke as vehicles and goods were on fire while vendors escaped leaving their goods behind." All the injured had been sent to hospital, the Xinjiang regional government's web portal Tianshan said.

The vast, resource-rich far-western region has seen periodic violence which has increased and sometimes spread beyond it in recent months.

Beijing says it faces terrorism from a violent separatist movement there, driven by religious extremism and foreign groups.

Critics say the security threat in Xinjiang is exaggerated by Beijing to justify hard-line measures, and instead point to economic inequality and cultural and religious repression of Uighurs as causes of unrest.

On April 30, the final day of a visit by President Xi Jinping to the region, assailants armed with knives and explosives carried out an attack at a railway station in Urumqi, killing one person and wounding 79. Two attackers also died.

The main plotter had formulated plans from abroad, then eight days before the incident ordered 10 people to make an explosive device and choose a target, Xinhua said in a later report.

In March attackers went on a stabbing spree at a railway station in the southwestern city of Kunming, killing 29 people and wounding 143 in an incident dubbed "China's 9/11" by state media. Four of the assailants were shot dead by police.

In 2009 ethnic riots erupted in Urumqi between Uighurs and the country's majority Han Chinese, leaving 200 people dead and prompting a security crackdown.

China has dramatically increased the number of armed patrols on its streets in response to the spate of violent incidents.

Thursday's blasts came a day after state media reported that courts in Xinjiang jailed 39 people for offences including spreading "terrorist videos".

The 39 were given prison sentences of up to 15 years, the state run China News Service said, adding that several had "organised, led and participated" in terrorist organisations.

Rights groups say the tensions in Xinjiang are driven by cultural oppression, intrusive security measures and immigration by majority Han Chinese which have led to decades of discrimination and economic inequality. Beijing says that its policies in the region have brought prosperity and higher living standards.