Kavkazcenter.com has been known to carry accurate claims of attacks
A North Caucasus Islamist group has claimed responsibility for a bomb that killed 26 people on a Moscow-to-St Petersburg train, a website says.
The website claim on Kavkazcenter.com said last Friday's attack was carried out by the "Caucasian Mujahadeen" on the orders of its leader, Doku Umarov.
He is described as one of Russia's most wanted rebels, but it was not possible to verify the claim's authenticity.
Moscow had earlier described the Nevsky Express attack as an act of terrorism.
Doku Umarov, a Chechen, is considered the leader of the Islamist insurgency in the North Caucasus. He says he is fighting to expel Russian forces, and to turn the region into an Islamic emirate.
Wednesday's web statement said Friday's attack was an "act of sabotage", and part of a series of operations targeting strategic sites in Russia.
"Today, we carry out sabotage operations on electricity transmission lines, oil-and-gas-wires. Many of the operations are under preparation status.
"We intend to conduct such diversions in future, which are the just acts of vengeance... These diversions will continue for as long as the occupants in the Caucasus will not stop its policy of killing ordinary Muslims purely on religious grounds."
Kavkazcenter.com has carried statements before by North Caucasus groups claiming responsibility for attacks on Russia that have turned out to be correct.
Vladimir Yakunin, the head of Russian railways, told the BBC that he thought there were similarities between Friday's derailment and a 2007 attack on the same line which injured 30 people, and for which two men from Ingushetia have been charged.
But he said he did not know exactly who was responsible for this latest incident.
On Tuesday, the funerals were held of some of the 26 who died on the Nevsky Express - a luxury high-speed train popular with government officials and business executives.
Among those buried were Sergei Tarasov, a former St Petersburg vice-governor, and Boris Yevstratikov, head of the Federal Agency for State Reserves.
Nearly 100 others were wounded after what police called an "improvised explosive device" derailed the train's last three carriages near the town of Bologoye, some 400km (250 miles) north-west of Moscow.
A second, less powerful device which went off on Saturday near the site of the first - reportedly triggered by a remote mobile - is said to have injured one of the investigators combing the train's wreckage.
Police have identified a house where they believe suspects had been staying, Russian media reports say, and have released a photo-fit of a man they believe is linked to the bombing.
The interior ministry circulated the sketch of a man in his 50s with a wig of red hair, who had reportedly been seen by witnesses near the blast scene.