A blast occurred early Wednesday at an illegally-run oil refinery in Shenyang, capital of Northeast China's Liaoning Province, leaving four dead and 12 others injured.

The city government source said the blast, happening at around 2:20 a.m., was triggered by a fire in the workshop of the Hongda Chemical Plant.

Workers of the plant refined oil from the disused chemical raw materials by heating them with fire, which is a refining method strictly banned by the government.

Seeing the temperature rising slowly, the workers used pitch as a fuel and then made the fire out of control, said the source.

Four workers were killed on the spot and the 12 injured have already been hospitalized, five of whom in a critical condition.

The Shenyang city government convened an urgent meeting Wednesday, reiterating to close down all the illegal chemical plants or refineries in the city.