Explosion ripped through an oil refinery and an emergency has been declared least 20 casualties

Workmen inside the building said the explosion sounded like a “sonic boom”.

The refinery had been shut down for maintenance when the explosion occurred – which was described as the “most dangerous time”.

Superior Police urged people to avoid the area and roads surrounding the plant were closed down.

Employees on the site have been evacuated from the scene as the fire crews battled the blaze.

At least 20 people have been injured in the explosion, but there are not believed to be an fatalities.

Seven ambulances rushed to the scene to deal with the wounded, who were taken to local hospitals.

Local people in the nearby town reported feeling the explosion.

Pictures showed huge plumes of smoke rising over the plant – with flames also visible.

Superior Police said: "Large incident in the area of Stinson and Hill. Do not enter this area. If evacuation is in order, we will let you know.”

And local schools urged parents not to worry – saying there is “no reason to evacuate the school buildings”.

They added: “We will be keeping all students in the buildings.”

Three fire engines were sent to the explosion at the plant, which happend around 10am local time.

Superior Police said: "Large incident in the area of Stinson and Hill. Do not enter this area. If evacuation is in order, we will let you know.”

And local schools urged parents not to worry – saying there is “no reason to evacuate the school buildings”.

They added: “We will be keeping all students in the buildings.”

Three fire engines were sent to the explosion at the plant, which happend around 10am local time.