Russian media report that at least 10 people have been killed and three others injured in an explosion at a railway station in the southern city of Volgograd, near the volatile North Caucasus region.
Officials say the casualty count could change as events develop.
Accounts say the blast occurred inside the station in the early hours of Sunday. The cause of the explosion was not immediately clear.
An attack in the same city by a female suicide bomber on October 21 killed six people, including the bomber, and wounded around 30.
Russia's Investigative Committee identified the suicide bomber as a 30-year-old resident of Dagestan, one of the republics in Russia's predominantly Muslim North Caucasus region.
In recent years, Dagestan has become the epicenter of an ongoing Islamist insurgency in the North Caucasus.
In early July, the leader of the Islamist insurgency in the North Caucasus, Doku Umarov, declared an end to a moratorium on attacks on Russian civilian targets he had announced the previous year.
He also called on insurgents to target the 2014 Winter Olympics set to take place next February in Sochi, not far from the North Caucasus.