Chechen rebel leader Doku Umarov, whose Caucasus Emirate group claimed responsibility for a string of deadly attacks in Russia over the last years, has been killed, a pro-insurgency website said Tuesday. "The leadership of the Caucasus Emirate officially announces the martyrdom of Emir Dokku Abu Usman," the Kavkaz Center website said, using the Arabised nom-de-guerre of Umarov. It gave no details of how he was said to have been killed and there was no confirmation from the Russian authorities who are usually keen to rapidly boast of the "liquidation" of top Caucasus militants. "Foreign media periodically publish information about the liquidation of Doku Umarov but the Russian special services at this moment do not have such information and will not comment on it," the National Anti-Terror Committee said in a statement to the state RIA Novosti news agency. But Kavkaz Center published a statement saying that the Caucasus Emirate had appointed a militant named as Sheikh Ali Abu Mukhammad as its new chief. The website also posted a video said to be of the bearded militant confirming the death of Umarov and outlining his leadership strategy for the Caucasus Emirate, which wants to impose an Islamic state across the mainly Muslim Northern Caucasus. Ali Abu Mukhammad was shown in battle fatigues at an undisclosed location with a gun behind him resting on a rock. He spoke in a mixture of Arabic and heavily-accented Russian. His origins and citizenship were not made clear. "I want to inform you that our brother (Doku Umarov) has left this world," he said. "Our condolences to his family and the Muslim world." "His soul is with the green birds in paradise," he added. Ali Abu Mukhammad said he did not consider himself worthy of the duty but had taken it on after being asked "by the brothers". "I will take on the responsibility myself," he said, saying the "jihad" by the Caucasus Emirate group would continue. -Feared militant leader- Umarov became head of the guerrilla movement in Chechnya in June 2006 after its previous leader Abdul-Khalim Saidullayev was killed by the Russian military. He was known as an ally of notorious rebel chief Shamil Basayev, who claimed responsibility for dozens of attacks, including the 2004 Beslan school hostage crisis in which more than 330 people were killed, many of them children. Basayev was killed by Russian forces in July 2006. The predominantly Muslim region declared independence from Moscow after the Soviet collapse in 1991 and then fought two bloody wars with the federal government before Moscow finally re-established control. Umarov served as secretary of Chechnya's national security council from 1997 to 1998, during a short-lived period between the two wars when Moscow tolerated the region's autonomy. As head of the Caucasus Emirate he claimed responsibility for numerous atrocities inside and outside the Caucasus. The Caucasus Emirate took responsibility for an attack on a Moscow-Saint Petersburg train in 2009 that left 27 dead. It claimed the Moscow metro suicide bombings of 2010 and the 2011 suicide attack on Domodedovo airport that left dozens dead. Like other rebel leaders, Umarov's death has been proclaimed several times by the Russian authorities. He was first declared killed in August 2000 during a special operation by Russian forces.