TORONTO (Reuters) - Five people have been injured, one critically, after an explosion at a tank car maintenance facility in Sarnia, Ontario, police and the city's mayor said on Saturday.

Police in the city, home to a number of oil refineries and petrochemical plants, said on Twitter the fire at the Veolia Environmental Services site had been quickly brought under control.

It said one of the five injured was being transferred to a London, Ontario hospital.

Sarnia Mayor Mike Bradley told the Canadian Broadcasting Corp one person had been critically injured, while three had serious injuries and one had minor injuries. The mayor said it was unclear exactly what caused the explosion.