At least six people were killed and 27 others wounded when a bomb exploded Monday on a bus in the Russian city of Volgograd, the National Counterterrorism Committee said.

Seven of the 27 people hospitalized are listed in serious condition.

"Today, in the city of Volgograd, a blast from an unidentified explosive device caused the loss of human life aboard a passenger bus," the Interfax news agency reported, citing a statement released by the counterterrorism agency.

A case has been opened on "terrorism, murder and arms trafficking" charges, National Counterterrorism Committee chairman Vladimir Markin said.

Bomb experts from the Federal Security Service, the former KGB, are gathering evidence at the scene of the explosion.

About 40 people were aboard the bus when the bomb exploded near a stop on Route 29 in Volgograd, which has a population of slightly more than 1 million. EFE