A video by the Islamic State (IS) group threatens attacks in Russia "very soon" in retaliation for Russian bombing in Syria, the SITE monitoring group says.
The video being circulated on social-media sites is in Russian with chants of "Soon, very soon, the blood will spill like an ocean" sung over graphic images of violence.
The video, which the Kremlin says security services are investigating, comes as Russian authorities have reported increased security concerns in Moscow and other major cities in recent weeks.
Moscow estimates that 5,000 to 7,000 people from former Soviet states are fighting for IS in the Middle East, raiising concerns about the threat of homegrown terrorism, including from returning fighters.
IS has previously called for attacks on Russia in revenge for strikes by their warplanes on its fighters in Syria.
Western intelligence officials suspect an IS affliliate planted a bomb in a Russian airliner that crashed in Egypt's Sinai Peninsula last month, killing all 224 people on board.
IS's Egypt affiliate, Sinai Province, claims to have brought down the plane.