Two workers were killed and another two injured when an oil storage tank exploded Tuesday at a refinery in northwest China's Xinjiang Uygur Autonomous Region, police said.

Four workers were doing welding operations on top of the 20-meter-high tank when it exploded at around 2:30 p.m. in Midong District of Urumqi, the regional capital of Xinjiang, said a city public security bureau spokesman.

Two workers died when they fell to the ground and the other two were injured, but described as stable.

It was unknown if the blast was connected to the welding, said the spokesman.

Police are investigating the cause of the accident.