At least 15 people have been killed and 23 injured after a powerful blast rocked the Russian city of Volgograd, a day after a suicide bombing decimated the central station of the same city.

The latest explosion, which took place on a trolleybus near a busy market in the district of Dzerzhinsky, is being treated as an act of terror, as it closely resembles the previous day's attack.

Reuters, citing state TV footage, reported that the roof of the blue and white bus was ripped off and debris was scattered all over the blast site.

"People were lying on the ground, screaming and calling for help," NBC News quoted witness Alexander Koblyakov as telling Rossiya-24 TV.

"I helped carry out a police officer whose head and face were covered in blood. He couldn't speak."

Another witness told Russia's ITAR-TASS "there was a loud 'pop', then a flash. Everything was enveloped in smoke."

Investigators have arrived at the scene to probe the attack. Irina Gogoleva, spokesperson for the South Regional Centre of the Russian Interior Ministry, said: "A blast happened inside a trolleybus in Volgograd, according to preliminary reports, 10 people died, 10 were wounded."

However, media reports later revised the death toll above Gogoleva's estimate.

Security on high alert

Security has been beefed up at important places across Russia. No group has claimed responsibility for either of the two attacks.

President Vladimir Putin has been briefed on the situation by authorities, the Kremlin said.

The latest bombings have raised concern over the safety of the upcoming Winter Olympics as Russia is set to host the event in February in the city of Sochi, west of Volgograd.

Russia Vladimir Putin