Police in China's troubled north-western region of Xinjiang have shot dead four suspects after 11 people died in one or more knife attacks and a possible explosion, according to state media.

Officers have detained another four and are hunting four others following the latest violence, in the Silk Road city of Kashgar.

Blasts were heard shortly before two men knocked down pedestrians with a hijacked truck and stabbed them in an assault late on Saturday, said official news agency Xinhua. It left eight dead – including one of the assailants – and 28 injured.

Three died in a separate incident on Sunday afternoon and 10 were injured, Xinhua reported, with police among the casualties. It said local sources initially blamed a blast, but witnesses said the victims were "hacked to death by rioters".

The deaths come less than two weeks after 18 people died in what Chinese authorities described as an attack on a police station in the region.

It is unclear what caused this weekend's incidents and whether they are related to ongoing ethnic tensions.

Xinjiang has seen several outbreaks of unrest and violence in recent years, with almost 200 people – mostly Han Chinese – killed in ethnic riots in the regional capital of Urumqi two years ago. Beijing has poured huge amounts into security and economic development in the region since then.

Officials blamed the 2009 riots on a small number of separatist terrorists. But many within the large Uyghur Muslim population chafe at religious and cultural restrictions and feel their way of life is being eroded by an influx of Han migrants. Others complain of discrimination from Han bosses and officials.

Xinhua said the first two blasts were heard at about 10.30pm on Saturday. One came from a parked minivan and another in a street crowded with food stalls.

At 11.45pm, two men jumped into a truck which had halted at traffic lights on the same street, stabbing the driver to death and making off with the vehicle.

Xinhua said that they struck several people as they drove off, while Xinjiang news portal Tianshannet.com.cn said the men drove into a crowd, left the truck and started attacking people, killing six.

Victims and others fought back, beating one attacker to death and captured the other, it added.

The attack has echoes of a 2008 attack on a border paramilitary police station in the city, in which assailants rammed a group of police recruits with a truck and then attacked them with knives, killing 16 officers.

One woman told Xinhua her colleague was killed as he tried to intervene. "Our security guards tried to save the residents while our manager attempted to subdue an attacker by holding him, but the man had a knife and stabbed him in his abdomen," said Yang Hongmei. "There were cries and blood everywhere ... Terrified people flooded into our office to hide."

Another man, Li Fu, said he was repairing motorcycles at the roadside when he saw a truck plough into a crowd. He thought it was a traffic accident, but then heard people screaming "someone is killing people", he said.

An Uyghur exile group alleged that at least 100 people had been arrested.

"The entire city of Kashgar is under martial law, and authorities have arrested at least 100 Uyghurs," the Germany-based World Uyghur Congress said in a statement sent to Reuters.

"There is no way to protest peacefully the Chinese suppression there, and the policy of calculated resettlement," added group spokesman Dilxat Raxit, referring to ethnic Han Chinese being relocated to live in Xinjiang.

A press officer for Xinjiang provincial government said they were still trying to determine details of the incidents.

A spokesman for Kashgar police said they could not comment.

Earlier this month, Xinhua reported that armed officers shot dead 14 "rioters" after they attacked a police station in Hotan, hacking a security guard to death, starting fires and taking six hostages.

A policeman and two civilians also died in the attack, it said.

The World Uyghur Congress alleged that security forces beat 14 people to death and shot dead six others.

It said the demonstrators took hostages to try to secure the release of local youths detained by authorities after earlier protests.