What police had originally reported as a traffic collision involving a bus, tractor and a car was in fact revealed to be a bomb attack gone awry in Wenzhou city of Zhejiang province, Xinhua news agency said.

Hu Baoqiang, 44, on Saturday drove a tractor carrying a tank of gas and explosives to a small building alleged to be a local gambling den, in what appeared to be a planned attack, Xinhua cited police as saying.

But the tractor collided with a car and a minibus outside the building, the news agency said, causing an explosion that left 17 people dead, including Hu.

Witnesses said the tractor hit the two other vehicles, which were stationary, as Hu tried to make his way around them, according to Xinhua.

Forty people were injured, including bus passengers, passers-by and people in the alleged gambling den.

Investigators found explosive-making materials in Hu's house, Xinhua quoted police as saying. The nature of his grievances against the gambling den were unclear.

Police are investigating how he got hold of the materials, the news agency said. -- AFP