Video: Aftermath of second Volgograd explosion

The destroyed trolleybus stands on a street in Volgograd after a bombing on the packed vehicle (STR/AFP/Getty Images)

Fourteen people have been killed in a bus explosion in Volgograd, a day after a suicide bombing left at least 17 dead at a railway station in the same Russian city.

The morning rush hour blast, which federal investigators have called a "terrorist act", reduced the trolleybus to a twisted, gutted carcass, its roof blown off and bodies and debris strewn across the street.

Russian investigators said they believe the explosion was caused by a male suicide bomber who detonated the bomb in the vehicle's passenger area.

"It is now possible to preliminarily say that the explosive device was set off by a suicide bomber - a man whose body fragments have been collected and sent for genetic testing," the federal Investigative Committee said in a statement.

The two explosions in close concession have put the city on edge and highlighted the terrorist threat that Russia faces, as it prepares to host the Winter Olympics in February.