The BBC's Martin Dawes in Nairobi: "Very inaccessible area"

The overnight passenger train travelling from Nairobi to the Indian Ocean port of Mombasa derailed at around 0400 (0100 GMT).

The Kenyan police, air force, and a host of medical groups have launched a huge emergency effort, treating those with minor injuries at the scene and flying the critically injured to the capital Nairobi as well as to hospitals in Mombasa and the nearby town of Voi.

Daniel Burstow: 'I felt the train was bouncing around'

Some reports say the crash may have killed as many as fifty passengers. The Kenyan defence department says there were at least five non-African foreigners among the dead, but their nationalities were not immediately available.

The accident happened at Man-eaters, a junction near Tsavo National Park, around 300 km (186 miles) south-east of the Kenyan capital.

It was not clear how many passengers were on the train but a railway company spokeswoman said the service, often used by tourists travelling to Kenya's coastal resorts, usually carried around 300 people.

BBC Nairobi Correspondent Martin Dawes says the region is very inaccessible and it is difficult to get information from the area.

Survivors: Doors locked

Survivors said the train doors were locked and they had to climb out of carriage windows.

"I was awake and I felt the train was bouncing around quite viciously - I immediately knew something was wrong," said 20-year-old British geography student Daniel Burstow at Nairobi Hospital.

"There was a big problem in that all the doors were locked and nobody could get out," Burstow said. "We had to break windows to get them [the survivors] all out."

Mr Burstow said all nine of his friends in his group survived, although many had light injuries.

"There were a lot of dead and dying," he said.

Injured passengers were taken to hospital in Mombasa and Voi

Another survivor, Michael Greenwood, 59, of Florida, told how he had to walk about a kilometre to the main Nairobi-Mombasa road where Chinese workers carrying out road reparis were the first to come to help the victims.

He said most of the damage appeared to have been done to the third-class cars towards the front of the train.

Mr Greenwood said he had been travelling in the last car of the train, which was not damaged, and was unhurt.

Heavy rains

The railway was constructed nearly a century ago by the British colonial authorities and has seen little modification over the decades.

The train service has become more popular since torrential rains in late 1997 caused heavy damage to the highway linking Nairobi and Mombasa - Kenya's two largest cities - entirely washing away two large sections of the road.

Heavy rains in recent weeks have washed out portions of the railway on the other side of Nairobi on the route to Kampala, Uganda.