They also threw explosives during the attack in the regional capital Urumqi. More than 90 people were injured, the state-run Xinhua news agency said.
The Ministry of Public Security called it a “violent terrorist incident”.
Xinjiang, which is home to the Muslim Uighur minority, has seen a spate of attacks in the past year.
Information about incidents in the region, where ethnic tensions between Uighurs and Han Chinese continue, is tightly controlled.
Pictures on Weibo microblogs – China’s equivalent of Twitter – appeared to show Thursday’s attack taking place at one end of a busy market street lined with vegetable stalls.
One of the two vehicles exploded.
A local shopkeeper told the Associated Press he heard “four or five explosions” and saw “three or four people lying on the ground”.
“Witnesses said two cross-country vehicles driving from north to south ploughed into people in the market at 07:50. Explosives were thrown out of the vehicles,” the Xinhua report said.
The injured were taken to several hospitals, Xinhua said.
The World Uyghur Congress said the authorities in the Chinese capital Beijing should not increase the crackdown in Xinjiang.
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