A suspected female suicide bomber has set off explosives on a bus in the southern Russian city of Volgograd, killing at least six people.
Officials believe the woman was from Dagestan in the North Caucasus and was the partner of an Islamist militant.
The blast, which happened just after 14:00 (10:00 GMT), also injured more than 20 people, some of them seriously.
An Islamist insurgency in the North Caucasus region has led to many attacks there in recent years.
The BBC's Steve Rosenberg in Moscow says the bombing has raised fears that militant groups may be planning to step up attacks in Russia in the run-up to the Winter Olympics in the Black Sea resort city of Sochi next February.
Via Pulp Ark