Chechen rebel leader Doku Umarov, the self-proclaimed emir of the Caucasus and Russia’s most wanted man, is dead, an Islamist militant website said Tuesday.

The Kavkaz Center said Umarov "became a martyr," but no further details were given, the BBC reported.

The militant group has claimed responsibility for a string of deadly attacks in Russia in the last few years. Umarov had called for attacks on the Sochi Olympics, but they were never attempted or were foiled.

A spokesman for Russia's national anti-terrorism body told state news agency RIA he could not confirm Umarov’s death, but Chechen Republic President Ramzan Kadyrov, a Kremlin ally, wrote on his Instagram: "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.

The Kavkaz Center website wrote on its Twitter account that Ali Abu-Mukhammad will now head the Caucasus Emirate. It did not say when or how Umarov had died, but the simultaneous release of a video address by his successor suggests it may have happened some time ago.

Over the last few years, the Russian authorities have announced Umarov’s killing several few times, but often recorded video statements just days after his alleged killing by the Russian forces.

One of Russia's most wanted rebels, Umarov has claimed to have ordered the January 2011 Moscow airport bombing that killed 36 people, and the March 2010 suicide bombings in the Moscow Metro, in which 39 people died. He is also said to have ordered the November 2009 bombing of a train from Moscow to St Petersburg that left 26 dead.