Two people were severely injured and two others have been reported missing after an oil residue tank blast caused a fire on Sunday in Liaoning province, firefighters said.

Two explosions were heard at an oil refinery in Dalian in Northeast China. After the blasts, heavy black smoke was seen billowing into the sky.

Firefighters have arrived at the refinery which belongs to the China National Petroleum Corporation. The fire is now under control. Two people have been injured and two are missing. The cause is not yet known.

One tank containing residual diesel oil exploded around 2:20 pm at a PetroChina outlet in the port city of Dalian, causing a nearby tank to burst into flames, firefighters with the city said.

More than 300 firefighters were mobilized to put out the fire by 4:30 pm.

The tank storage hit by the blast was part of the PetroChina Dalian Petrochemical Company and was being renovated so production could resume later this year, according to an official at the company. PetroChina is the country's top oil and gas producer.

Four people were at the site at the time of the accident. The injured have been hospitalized, and doctors said they were both in life-threatening conditions.