The death toll from Monday's suicide bombing attack on a trolleybus in Russia's southern city of Volgograd has increased to 15, a medical source in the Southern Federal District told Interfax.

"The number of those killed in the terrorist attack on a trolleybus currently stands at 15. All the victims have already been identified," he said.

Twenty-seven people injured in the blast are being treated at hospitals, he said. One of them is in critical condition, and another eight are in serious condition.