An explosion on a bus Monday in the southern Russian city of Volgograd killed at least six people and injured some 20 others, law enforcement authorities said, dpa reported.
Russia's Anti-Terror Committee said the blast was caused by an unidentified explosive device, the Interfax news agency reported.
A local Investigative Committee official told the agency that the bodies of six victims had been recovered.
A police spokesperson said seven of the injured were in critical condition.