* Website publishes obituary of rebel leader
* Video shows man who says he is the new leader
* New leader of insurgency native of Dagestan (Adds analysts quote)
By Alissa de Carbonnel
MOSCOW, March 18 (Reuters) - Russia's most wanted man, Doku Umarov, is dead and has been replaced as the leader of an Islamist insurgency in the North Caucasus, a website that sympathises with the militants said on Tuesday.
Umarov's forces have claimed responsibility for several deadly bombings, including in Moscow. His death has been reported several times by the Kremlin-backed leader of the Chechnya region, but never before by Umarov's sympathisers.
The Kavkaz Centre website issued an obituary, calling Umarov a martyr who had "given 20 years of his life to the Jihad."
It did not say when or how he had died, but the simultaneous release of a video address by a militant introducing himself as Umarov's replacement indicated it may have been some time ago.
The death of the veteran Chechen rebel, if confirmed, marks a success for President Vladimir Putin in his attempts to end the violent struggle against Russian rule.
"The departure of Doku marks the end of the older generation of rebels, who began as separatist in Chechenya," the International Crisis Group think tank's North Caucasus director Yekaterina Sokirianskaya said.
A bearded man named as Ali Abu Mukhammad says in the video posted on You Tube that he was asked to take Umarov's place.
"I am declaring that I am taking on this responsibility," he says, wearing combat fatigues and sitting on a grassy slope with an automatic rifle at his side and a black-and-white Islamist flag behind him.
A spokesman for Russia's National Anti-Terrorism Committee was quoted by state news agency RIA as saying he could not confirm Umarov was dead.
But Chechen leader Ramzan Kadyrov, who is backed by Moscow, wrote on his Instagram microblog: "The terrorist mouthpiece reports that Doku Umarov is dead!"
"Umarov was killed in a security operation, which I wrote about earlier," he wrote. "Now it is confirmed by the rats themselves."
Umarov, a former Chechen rebel who embraced the jihadist goal of establishing a caliphate, united local militant groups in the Chechnya, Dagestan and other North Caucasus provinces under his command in 2007.
Listed as a terrorist group by the United States in 2011, his Caucasus Emirate group said it was behind suicide bombings that killed 37 people at a Moscow airport in 2011 and 40 people on the Moscow subway in 2010.
The group is also widely thought to have masterminded three deadly bombings that killed over 40 people in the southern Russian city of Volgograd in as many months, before the 2014 Sochi Games.
Umarov, who styled himself as the Emir of the Caucasus Emirate, had urged his fighters in a video posted online last July to use "maximum force" to prevent Putin staging the Olympics.
Experts said his death may have helped prevent an attack by Umarov's group on Russia's hosting of the Winter Olympics in Sochi but were sceptical it would weaken the insurgency in the long term.
"He was a poster boy used to recruit more and more young people under the banner of Jihad," Alexei Filatov, deputy head of the veterans' association of the Alfa anti-terrorism unit.
"His real role in the leadership of the terrorist movement was small."
Umarov's replacement by a native from Dagestan, a hub of Islamist militancy on the Caspian where Boston Marathon bombing suspect Tamerlan Tsarnaev spent the first half of 2012, cements the insurgency's transformation to a separatist to an Islamist fight.
"Overall, Dagestan rebels have always been tougher than Chechen rebels," Sokirianskaya said. "It will be a more battle ready, more ruthless cell, even more integrate into the global jihad."
Putin crushed separatists in Chechnya when he rose to power 14 years ago. But the Islamist insurgency that spread to neighbouring Dagestan remains the deadliest conflict in Europe and still has the power to recruit fighters from as far afield as Canada.
The ranks of insurgents in the region are now filled by youths disillusioned by police brutality, joblessness, corruption and the perceived persecution of religious conservatives in the mainly Muslim province. (Additional reporting by Alexei Anishchuk; Writing by Alissa de Carbonnel; Editing by Tom Heneghan)