A huge falling rock derailed a train in the French Alps today, killing two people and partly pushing one of its carriages over the edge of a mountain slope, local officials said.

At least seven people were injured, one of them seriously, in the accident, which occurred on Saturday morning on a train line popular with tourists between the villages of Saint-Benoit and Annot in southeastern France, the officials said.

"An enormous rock broke off from the mountain and hit the side of the train with extreme force," Charbel Aboud, deputy prefect for the region, told a news conference, adding it may have weighed as much as 20 tonnes.

The two dead were a French woman from the region and a Russian woman who was travelling with her husband, he said.

Officials said it was unclear what had caused the rock to fall. Netting is used in the region to prevent rockfalls and this was the first such accident known on the line, which runs from Nice on the coast to the Alpine town of Digne-les-Bains.

The mountainous location and snow on the ground had initially hampered rescue efforts.

Seven people were killed last year when a mainline train derailed south of Paris in an accident that has been attributed to a fault on the track.