The explosion happened not far from Chechnya

Government officials said they believed it was an act of terrorism.

It is thought that two bombs operated by remote control went off beneath the train.

There are reports that a man was arrested after trying to escape from the scene immediately after the incident.

Everything shook from side to side - all that I can remember is lots of dust all over the place and everything shaking Victim Ilya Kamyshanov

He is said to be in hospital with severe injuries.

The train was travelling in the North Caucasus between Mineralniye Vody and the spa town of Kislovodsk.

The blast occurred just before 0800 (0400 GMT) near Podkumok station.

It tore through the floor of the train killing three people outright. One more later died in hospital.

All the dead were between the ages of 14 and 22.

Earlier reports said five people had died. Itar-Tass news agency said a fifth victim was taken to hospital presumed dead but doctors managed to save her.

"Everything shook from side to side," Ilya Kamyshanov, who was injured in the blast, told AFP news agency. "All that I can remember is lots of dust all over the place and everything shaking."

Emergency workers are at the scene, not far from the breakaway republic of Chechnya.

Attacks planned

Russia has recently been rocked by bombings and other attacks, which the government blames on Chechen rebels.

Police right across the region were already on high alert after reportedly receiving intelligence information that rebels were planning a series of attacks across southern Russia.

The blast coincides with a visit by Russian President Vladimir Putin to the southern city of Rostov for a meeting of the country's State Council.

It also comes on the first day of campaigning for Russia's parliamentary elections.

The BBC's Sarah Rainsford says that if there is a Chechen link this time, it will be another blow to Mr Putin and his party.