“One of our brothers who has never been to the Caucases took up the oath of (former independent Chechen president Doku Umarov) and expressed his desire to execute the damned Sysoyev,” said a statement on the Kavkazcenter.com website.
Daniil Sysoyev, 35, was killed on November 20 when masked gunman walked into Saint Thomas’s church in southern Moscow and shot him four times.
Doku Umarov emerged as the leader of the remaining active rebel movements in the North Caucases in 2007 and is considered enemy number one in the region by Russian authorities.
The statement on the Kavkazcenter.com website, which is often used by militants, accused Sysoyev of writing several pamphlets insulting Islam.
It warned “those in the future who defame Islam and insult the religion of Allah will suffer the fate as Sysoyev.”
Sysoyev, who was criticised by Muslim organisations for his statements on Islam, had reportedly contacted Russian security services several times over threats.