Islamist rebels claim Russian rail bombing

"This operation was prepared and carried out...pursuant to the orders of the Emir of the Caucasus Emirate Doku Umarov," the website said, quoting a letter it said it received from Islamist rebels.

Umarov is Russia's most wanted guerrilla leader and leads an insurgency which aims to free the mainly Muslim North Caucasus from Moscow's rule.

Friday's attack against the luxury Nevsky Express train running between Moscow and St Petersburg was part of a campaign of sabotage against strategic economic targets, the letter added.

"These acts of sabotage will continue for as long as those occupying the Caucasus do not stop their policy of killing ordinary Muslims," the letter added.

Friday's bombing was the worst terror attack in Russia outside the North Caucasus in five years. It raised fears of a new wave of attacks in major Russian cities.

The KavkazCenter website has in the past carried claims of responsibility for rebel attacks which law enforcement officials have subsequently said are not genuine.

(Writing by Conor Humphries; Editing by Michael Stott)