There was no immediate information about who might have carried out the bombing or why. The anti-terrorism agency, which is part of the Federal Security Service, said investigators were on the scene.
A total of 40 people were on the city bus at the time of the explosion, said Irina Gogolyeva, a spokeswoman for the Emergency Situations Ministry. She said at least five died and 17 were injured.
Many of the injured were hospitalized in serious condition, the state news agency ITAR-Tass reported.