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 blast was caused by "an unspecified explosive device," the National Anti-Terrorism Committee said in a statement.

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.