Chinese police have shot dead three people who allegedly attacked a Communist Party office in the restive Xinjiang region on Wednesday, state media said, in the bloodiest such incident in months.

The three "rioters" drove up to a local party office in Moyu County and "detonated (an) explosive device", killing two and injuring three others, the official Xinhua news agency reported, citing a statement from the local authorities.

The three assailants were then "shot dead by police", Xinhua added, without specifying whether they were killed on the spot or later in the day.

A previous account had said that four attackers were shot dead after killing one civilian.

The brief report described the assault as a "terrorist attack".

China tends to reserve the "terrorist" label for attacks involving the mostly Muslim Uighur ethnic minority.

The far-western region of Xinjiang is the homeland of the Uighurs -- many of whom complain of discrimination and controls on their culture and religion -- and is often hit by deadly unrest.

Beijing regularly accuses what it says are exiled Uighur separatist groups such as the East Turkestan Islamic Movement of being behind attacks in the resource-rich region.

But overseas experts doubt the strength of the groups and their links to global terrorism, with some saying China exaggerates the threat to justify tough security measures.

In November 2015, police killed 28 members of a "terrorist group" over the course of a 56-day manhunt following an attack on a colliery in Aksu two months earlier that left 16 people dead.

In March 2014, 31 people were knifed to death at a train station in Kunming, in southwestern China, with four attackers killed, with Xinjiang separatists blamed and state media dubbing it "China's 9/11".

Two months later 39 people were killed in a bloody market attack in Urumqi, the capital of Xinjiang.