A US refinery has sent plumes of orange and black smoke into the air after a power failure shut the plant down.

Authorities in Colorado are telling people within a 2.5km radius of the Suncor Energy, Inc plant to remain indoors.

The company said the plumes, seen from downtown Denver, were “visible emissions” from the refinery in Commerce City which spokeswoman Nicole Fisher described as being from an uncombusted hydrocarbon. However, the Denver Fire Captain Greg Pixley said Suncor notified authorities that it was releasing sulphur dioxide, a gas created by the burning of sulphur.

The Colorado Department of Public Health and Environment said officials don’t know enough yet about what was in the smoke to determine if it was hazardous.

Fisher said Suncor is monitoring the air around the refinery, which produces petrol, diesel fuel and asphalt.

The power outage occurred around noon on Friday and the smoke dissipated by mid-afternoon.