A bomb blast rocked a passenger bus in the southern Russian city of Volgograd on Monday, killing at least five people and injuring 17 others, officials said.
The explosion was caused by "an unspecified explosive device," the National Anti-Terrorism Committee said in a statement. Interfax, a Russian news agency, reported that Russian intelligence officials believe it was caused by a female suicide bomber.
Officials said the blast occurred at around 2 p.m.
Volgograd is about 560 miles southeast of Moscow and has a population of about 1 million.
There was no immediate information about who might have carried out the attack or why. The anti-terrorism agency, which is part of the Federal Security Service, said investigators were on the scene.
Forty people were on the city bus at the time of the explosion, said Irina Gogolyeva, a spokeswoman for the Emergency Situations Ministry. Many of the injured were hospitalized in serious condition, the state news agency ITAR-Tass reported.
Security remains a concern throughout southern Russia before next year's Winter Olympic Games in the Black Sea resort of Sochi, which is next to the restless North Caucasus region.
Al Jazeera and wire services