Deadly bomb strikes Russia's Volgograd

At least 15 people have been killed and 23 wounded in an explosion on a packed trolleybus in Russia's Volgograd city, according to the Interfax news agency.

Volgograd was on high alert on Monday following the latest attack, that came a day after a suicide bomber attacked a train station in the city killing 17 people. 

"An explosion in Volgograd trolleybus is likely to be a terror attack, judging from the two previous attacks on a bus and a train station," said an official from the security services who was not named by Interfax.

The National Anti-Terrorism Committee said the trolleybus explosion was most likely caused by a bomb, but there were no further details.

Russian investigators have opened a criminal probe into a suspected act of terror as well as the illegal carrying of weapons, Investigative Committee spokeman Vladimir Markin told Interfax.

A suicide bomb attack killed at least 17 people and Volgograd's main railway station on Sunday.

The attack on Monday was the third attack on the city in the past three months. 

On October 21, a female suicide bomber blew herself up on a bus, killing six people.

The blasts have raised safety concerns in the region ahead of the Winter Olympics 2014 due in February in Sochi, about 650 kilometres southwest of Volgograd.