Twenty people died when a group armed with knives attacked a market in Xinjiang, the latest outbreak of violence in the ethnically divided Chinese region, authorities said Wednesday.

The motive behind the attack late on Tuesday was not immediately clear, but Xinjiang -- a vast region home to the mainly Muslim Uighur minority -- has suffered repeated outbreaks of ethnic unrest in recent years.

The Xinhua state news agency initially put the toll at 12 dead, including two assailants who it said were armed with knives.

But the official information website Tianshan said Wednesday the death toll was 20, including 13 "innocent people" and seven "terrorists" who were killed by police.

"Nine violent terrorists rushed into the crowd with knives, killing 13 innocent people and wounding several others," Tianshan said.

A police officer told AFP by telephone earlier that around a dozen Uighurs wielding axes attacked the market in the remote town of Yecheng, killing 10 people and police then shot five of the attackers dead.

Xinjiang, which borders Pakistan and Afghanistan, is home to around nine million Uighurs who complain of oppression under Chinese rule.

The number of Han -- China's dominant ethnic group -- living in the region has increased dramatically over the past decade, which government critics say results from a policy of migration to dilute any Uighur nationalist tendencies.

"At around 5.30 or 6.00 pm around a dozen rioters carrying axes appeared in the market, in an area that is mostly populated by Han people," said the police officer, who gave only his surname Tuo.

"Most of the victims were Han people, but some were Uighurs. Five rioters were shot, not two, and they were all Uighurs."

Xinjiang has been under heavy security since July 2009, when Uighurs launched attacks on Han people in the regional capital Urumqi.

The government says nearly 200 people were killed and 1,700 injured in the violence, which shattered the authoritarian Communist Party's claims of harmony and unity among the country's dozens of ethnic groups.

Many Uighurs remain angry at the harsh crackdown that followed the violence.

The government blames much of the violence in the resource-rich region on what it calls the three "evil forces" of extremism, separatism and terrorism.

But some experts doubt terror cells operate in Xinjiang, where the Turkic-speaking Uighurs practise a moderate form of Islam.

The World Uighur Congress, a German-based exile group, said authorities in Yecheng had detained more than 100 people and closed off all exits and entries in response to the latest attack.

Seven of those killed on Tuesday were Chinese armed patrol personnel, it said, citing local sources.

"The incident happened because Uighurs can no longer bear China's systematic repression, and are using primitive fighting methods to resist," said a statement by Dilxat Raxit, spokesman for the group.

He said armed police shot and killed 10 Uighurs and wounded 11 others and blamed a "huge influx" of Han Chinese immigrants and official discrimination against Uighurs for the violence.

"Uighurs have been completely stripped of any peaceful ways of resisting," he said.

Barry Sautman, associate professor of social science at the Hong Kong University of Science and Technology, said it was not clear whether the outbreaks of violence had increased since 2009, but that they appeared to be growing more daring.

"Since 2009, these incidents have been more, how should I say, spectacular, like storming police stations and attacks on police," he said.

Authorities in Xinjiang said last month they plan to recruit 8,000 extra police officers as China strengthens security in the run-up to a major leadership transition that begins later this year.

Chinese foreign ministry spokesman Hong Lei blamed "terrorists" for the latest attack although he added that "we do not yet know clearly who is behind these actions".

"The overall situation in Xinjiang is quite good," he told a regular briefing.

Timeline of unrest in China's Xinjiang region
Beijing, China (AFP) Feb 29, 2012 - At least 12 people have been killed in a fresh outbreak of violence in China's volatile northwestern region of Xinjiang, home to the mainly Muslim Uighur minority.

Here is a chronology of key events related to the restive region since 2008:

2008

August 4 -- China says "terrorists" kill 17 police officers and border guards in the far-western city of Kashgar, days before the Beijing Olympics open.

August 10 -- At least 10 attackers and one security guard are killed in bombings and a shoot-out with police in the remote city of Kuqa.

2009

June 25 -- A huge brawl erupts in the city of Shaoguan in southern China's Guangdong province between Uighur and ethnic Han factory workers. Two Uighurs are reported killed and dozens injured.

July 5 -- Uighurs gather in Urumqi to protest over the Shaoguan incident but violence erupts after security forces move in. Hundreds, perhaps thousands, of Uighurs riot.

July 6 -- Chinese security forces begin to pour into Urumqi and fan out across Xinjiang. The Xinjiang government blames exiled Uighur dissident Rebiya Kadeer for orchestrating the unrest. Kadeer, other Uighur exiles and Uighurs in Urumqi blame Chinese authorities for provoking the violence.

July 7 -- The government says nearly 200 died in the unrest, with more than 1,600 injured and hundreds arrested.

September 2 -- Han residents take to Urumqi's streets for several days of protests calling for a crackdown over a wave of syringe stabbings. The government eventually says nearly 500 were stabbed. Beijing blames "ethnic separatist forces". At least 75 are later reported arrested for the attacks.

November 9 -- China says it had put to death the first nine people over the July unrest. Eventually at least 26 are reportedly sentenced to death.

2010

March 15 -- Xinjiang's hardline top leader Wang Lequan says China will accelerate development in Xinjiang to quell unrest. State media later say more than one billion dollars will be invested.

April 24 -- Chinese media says the unpopular Wang has been replaced by Zhang Chunxian, a Communist Party official from Hunan province, in a move widely seen as a bid to placate public anger.

June 24 -- Police say they busted a Xinjiang "terrorist" ring behind a string of deadly attacks in the region, arresting at least 10 people.

2011

July 5 -- Rights group Amnesty International condemns China's crackdown on Xinjiang's Uighur population following the July 2009 violence, saying hundreds have been detained and prosecuted since the riots, with several dozen sentenced to death or executed and many more sentenced to long prison terms.

July 18 -- Police kill 20 protesters in clashes in south Xinjiang's restive city of Hotan, exiled Uighur groups say. State press say police fired on demonstrators after they had attacked a police station killing one policeman.

July 31-August 1 -- Two violent attacks by alleged terrorists leave 13 people dead in a Han Chinese section of Xinjiang's Kashgar city, while police kill eight suspected Uighur separatists. State press reports the suspects were trained in terrorist cells in neighbouring Pakistan.

September 15 -- Courts in Xinjiang sentence to death four Uighurs suspects convicted of involvement in the July 18 incident in Hotan and the July 30 and 31 violence in Kashgar.

December 28 -- Police shoot and kill seven "terrorists" in a hostage standoff that left one policeman dead in Xinjiang's Pishan county. State press reported the suspects were "terrorists" engaged in a "holy war."

2012

February 28 -- Rioters armed with knives kill at least 10 people in Yecheng, a town in the Kashgar region, while police shoot two of the attackers dead, state press says.