The explosion has been reported at one of Italy’s biggest oil refineries in Sannazzaro de” Burgondi, near Pavia, about 40km south of Milan.
Emergency plans have been activiated as residents are urged to stay in doors amid fears thousands of tonnes of fuel could go up in flames.
There is the risk of toxic smog which could spread across the region.
The Department of Civil Protection in the Province of Alessandria says nearby emergency centers have been activated.
The blast at the Eni refinery generated a fireball belching black smoke which can be seen for miles around.
There are currently no reports of an casualties.
A statment from Eni said the company is working with firecrews to bring the blaze under control.
The spokesman added they are now investigating the cause of the explosion.
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