A bomb ripped through a bus in the southern Russian city of Tolyatti on Wednesday killing at least seven people, including a child, police said.

The force of the blast at a road junction blew out the windows of a nearby residential block, the RIA-Novosti news agency quoted an official with the local rescue service, Andrei Derbenev, as saying.

Some 40 passengers, passers-by and nearby residents were injured by the explosion in Tolyatti, home to about 700,000 people and a centre of the Russian car industry, Derbenev was quoted as saying.

One medical official at the scene said the toll had risen to eight people.“There is reason to believe that an explosive device was attached either to the bottom of the bus or to the floor inside,” Interfax news agency said, quoting a Tolyatti police official.

Another source in the security services, quoted by ITAR-TASS, said the bomb might have been carried onto the bus by a passenger or could have been laid on the road in front of the bus.

It was the second blast on a bus in Russia in just over a week, following an explosion that killed one person and wounded five in the turbulent province of Dagestan on October 23.

While Russia continues to face insurgent violence in the Caucasus region, the Samara province is not known for such attacks.