Three dead in Scotland passenger train derailment: police
"Very sadly despite the best efforts of paramedics, we can confirm that three people have been pronounced dead at the scene," British Transport Police said in a statement.
The crash, close to the town of Stonehaven around 25 kilometres (15 miles) south of Aberdeen, followed severe flooding across parts of the region overnight.
Smoke could be seen rising from the scene in pictures broadcast on television news channels.
Union TSSA said the train was the 06:38 (05:38 GMT) service travelling from Aberdeen to Glasgow.
Sturgeon added that the incident happened in an area where it was difficult for emergency services to access.
British Prime Minister Boris Johnson said: "I am saddened to learn of the very serious incident in Aberdeenshire and my thoughts are with all of those affected. My thanks to the emergency services at the scene."
Thunderstorms and torrential rain battered parts of central and eastern Scotland overnight, creating hazardous travelling conditions.
Photos posted on social media by local authorities showed heavy flooding in the nearby town of Stonehaven early Wednesday.
Network Rail Scotland, the company which looks after the rail infrastructure, said it was working alongside emergency services.
"It is too early to confirm the exact nature and severity of the incident and more details will be made available once known," it said.
Moments after the emergency services had been notified of the accident, Network Rail reported there had been a landslip close by.
The local arm of Britain's National Health Service said it was setting up "major incident support" in Aberdeen for relatives and friends of anyone involved in the incident.
The BBC reported that one casualty was airlifted by helicopter to a nearby hospital.
Local MP Andrew Bowie said a "major emergency service response" was underway and he had spoken to the British transport minister, Grant Shapps, about the incident.
Shapps added on Twitter that he was in touch with Network Rail and other agencies and vowed "the UK Government will provide every support".
Britain's last major rail derailment was in 2007 in Cumbria, in northwest England, when a train on the main west coast line left the tracks, killing one passenger and seriously injuring 30 others.