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 sympathizes with the militants said on Tuesday.
If confirmed, his death is a breakthrough for President Vladimir Putin in his attempts to end the violent struggle against Russian rule.
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 sympathizers.
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.
A bearded man, identified 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 ... Now it is confirmed by the rats themselves," he wrote.
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.
His Caucasus Emirate group said it was behind suicide bombings that killed 37 people at a Moscow airport in 2011 and 40 on the Moscow subway in 2010.
The group is also widely thought to have masterminded three deadly bombings that 41 in the southern Russian city of Volgograd in as many months, before Russia hosted the Winter Olympics in the Black Sea city of Sochi.
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. (Reporting by Alissa de Carbonnel; Editing by Timothy Heritage)