An Islamic militant group said in a video posted online that it was behind two suicide bombings that killed at least 34 people last month in the Russian city of Volgograd, and threatened to attack the Sochi Winter Olympics, Reuters reported.
In a warning to President Vladimir Putin over next month's Games, a man says in Russian in the video: "If you hold the Olympics you will receive a present from us ... for you and all those tourists who will come over.
"It will be for all the Muslim blood that is shed every day around the world - be it in Afghanistan, Somalia, Syria, all around the world. This will be our revenge," he says.
The video says two men called Suleiman and Abdurakhman carried out the Volgograd attacks on behalf of a group known as Vilayat Dagestan and linked to an Iraqi faction called Ansar al-Sunna.
Dagestan is in Russia's North Caucasus, where militants are waging an insurgency to create an Islamist state. Doku Umarov, a militant leader, has urged the insurgents to attack the Games in Sochi, which lies on the western edge of the Caucasus mountains.
Putin has staked much personal and political prestige on the Games, which are intended to show how far Russia has come since the collapse of the Soviet Union in 1991.