At least 45 people have been killed after a train derailed outside the northern Spanish city of Santiago de Compostela, according to state media and eyewitnesses.

Train carriages lay on their sides with smoke billowing from the wreckage, photographs published on the Voz de Galicia newspaper website showed.

The head of Spain's Galicia region said that at least 35 people had been killed in the train derailment, although it was too soon to say what had caused the accident.

Eyewitnesses said that the derailed train, which was reportedly carrying 240 passengers when the accident happened, had caught fire after the crash.

'It was going so quickly ... It seems that on a curve the train started to twist, and the wagons piled up one on top of the other,' passenger Ricardo Montesco told Cadena Ser radio station.

'A lot of people were squashed on the bottom. We tried to squeeze out of the bottom of the wagons to get out and we realised the train was burning ... I was in the second wagon and there was fire ... I saw corpses,' he added.

Another witness told the station they had heard an explosion before seeing the derailed train.

Emergency services and police were not immediately available for comment.

Santiago de Compostela, the birthplace of Spanish prime minister Mariano Rajoy, is best known as the destination of an ancient Catholic pilgrimage.