The train's driver and conductor are among the fatalities, while six people have been taken to hospital with a range of injuries.

Dark smoke billowed from the stricken ScotRail train at the bottom of a narrow valley near Stonehaven, just south of Aberdeen, after it came off the tracks.

Two air ambulances and about 30 emergency service vehicles could be seen in a field just above the site of the derailment, which appeared to be difficult to access.

"My deepest condolences are with the loved ones of those who lost their lives in this tragic incident," said Sturgeon.

British Prime Minister Boris Johnson said he was saddened to learn of the "very serious incident" and his thoughts were with all those affected.

Stonehaven and the surrounding area had been hit by floods in recent days and some reports suggested a landslide may have played a part in the derailment, although that was not confirmed by the authorities.

Britain has one of Europe's lowest rates of fatal rail accidents, with a stronger safety record than Germany or France in recent years, according to Eurostat data that includes unauthorised people on railway tracks and at level crossings.

The worst rail disasters in Britain in recent decades were a 1999 collision between two trains at Ladbroke Grove, in London, in which 31 people died, and a 2001 accident near Selby in Yorkshire, northeast England, in which 10 people died after a car ran onto the track and was hit by two trains.

Australian Associated Press