At least five people have died after a bomb tore through a passenger bus in southern Russia, raising security fears four months before the Winter Olympic Games.
Officials say the blast occurred in the Volga River city of Volgograd, about 900 kms southeast of Moscow.
Russian news agencies report investigators suspect a female suicide-bomber.
Footage broadcast on state television shows a green city bus standing mangled in the middle of the street, its windows blown out on the left side.
The chief spokesman for the Investigative Committee, Russia's equivalent to the US Federal Bureau of Investigations, said officials had opened a formal terror probe.
Officials said the packed bus was carrying about 40 passengers at the time of the explosion.
Cities to the south, in Russia's North Caucasus, have seen attacks by Islamist militants in recent years.
Security remains a concern throughout southern Russia before the Winter Olympic Games in the Black Sea resort of Sochi, which is next to the North Caucasus region.
