Russia's most wanted man, Doku Umarov, is dead and has been replaced as leader of the armed rebellion in the North Caucasus, a prominent Chechen website has said.
If confirmed, the death of the self-styled 'Emir of the Caucasus Emirate' would be a major 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 attacks, including suicide bombings that killed 37 people at a Moscow airport in 2011 and 40 on the Moscow subway in 2010.
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 fighter 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 said.
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, who embraced the goal of establishing a caliphate, united local armed groups in Chechnya, Dagestan and other North Caucasus provinces under his command in 2007.
The group is widely thought to have masterminded three deadly bombings that killed 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 had urged his fighters in a video posted online last July to use "maximum force" to prevent Putin staging the Olympics.