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Investigators and rescue workers are seen amid damaged coaches at the site of the derailment. ((Ivan Sekretarev/Associated Press))

A North Caucasus Muslim rebel group has claimed responsibility for last week's Russian train bombing that killed 26 people, according to a website sympathetic to the militants.

The claim on the Kavkazcenter.com site said last Friday's attack was carried out on behalf of Doku Umarov, a Chechen separatist considered the leader of the Muslim extremist insurgency in the North Caucasus.

"We declare that this operation was prepared and carried out … pursuant to the order of the Emir of Caucasus Emirate," or Umarov, the statement on the website said.

The website has previously published statements by North Caucasus groups claiming responsibility for attacks that were later confirmed.

Umarov is one of Russia's most wanted rebels, and is thought to head a network of separatist cells across the mainly Muslim North Caucasus region. He has said he is fighting to expel Russia from the region and create an Islamic emirate.

Russian authorities characterized last week's attack as a terrorist attack but have not made any arrests. Police on Monday released a computerized sketch of a suspect.

Alexander Bastrykin, the country's top investigator, said in comments published on Wednesday in state newspaper Rossiiskaya Gazeta that the bombing bore the hallmarks of southern separatists.

Authorities said they found evidence that a bomb caused three carriages of the 14-car Nevsky Express, travelling from Moscow to St. Petersburg, to careen off the tracks Friday night as the train approached speeds of 200 km/h in a rural area.

Government officials were among the 26 people killed in the attack, and scores were injured.

It was the second attack on the train line in two years. A 2007 blast injured dozens but was not fatal.

With files from The Associated Press