Islamist militants from the North Caucasus on Wednesday claimed responsibility for the bombing of a Russian train that killed 27 people last week, according to a Chechen Islamic website.

The attack was prepared and carried out on the orders of "Emir of the Caucasus Emirate Doku Umarov," said the website (KavkazCenter.com), citing a letter it received from Islamist militants.

Umarov is Russia's most wanted guerrilla leader who leads an insurgency in the North Caucasus with the aim of establishing Islamic Sharia rule in the region.

Friday's bombing against the Nevsky Express train running between Moscow and St. Petersburg was the worst terror attack in Russia outside the North Caucasus in five years.