A bomb blast has ripped a trolleybus apart in the southern Russian city of Volgograd, killing at least 14 people in the second deadly attack in the city in two days.

The Monday morning rush-hour bombing, which left mangled bodies in the street, underscored Russia's vulnerability to militant attacks less than six weeks before the Sochi 2014 Winter Olympic Games.

State TV footage showed the twisted, gutted remains of the blue-and-white trolleybus, similar to a tram, with its roof blown off and debris strewn around the street, with Russian media reporting that children are among the victims.

Health ministry spokesman Oleg Salagai told Russian state television that 14 people were killed in the attack, while 28 others were wounded.

It came less than 24 hours after a suicide bomb blast killed at least 17 people and wounded more than 40 others at Volgograd's main railway station, a major transport hub in southern Russia.

President Vladimir Putin was briefed on the trolleybus attack by Federal Security Service chief Mikhail Bortnikov, a Kremlin spokesman said.

The national anti-terror committee announced Mr Putin had ordered security stepped up across all of Russia, for the second day in a row.

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Investigators have revealed the explosion on the packed electricity-powered commuter bus was detonated by a male suicide bomber.

"The explosives were detonated by a male suicide bomber, fragments of whose body have been found," the investigative committee said.

Investigators also said the kind of explosive used showed the trolleybus attack and the train station strike could have been jointly planned.

Local journalist Leonid Ragozin says the explosion occurred at about 8.30am (local time) while the bus was stopped at a market.

"This market is also near the hospital where people were taken... who were injured in the previous blast, in the blast that took place yesterday at the train station in Volgograd," he said.

Insurgent leader urges terror attacks to halt Olympics

Officials initially said the bomber behind Sunday's train station attack was a lone woman, however those claims have now been replaced by uncertainty about who was behind the explosion.

Two days earlier a car bomb in another southern city left three dead and in October another female suicide bomber killed six in an attack on a bus in Volgograd.

The attacks have sparked fears of a violent campaign by Islamic insurgents ahead of the Olympics.

However, Mike Tancred from the Australian Olympic Committee (AOC) says it is confident the team will be safe in Russia and the blasts will not affect preparations for the February event.

He says there are no Australian athletes in Russia at the moment.

Insurgent leader Doku Umarov, a Chechen warlord, has urged militants to use "maximum force" to prevent Russia from staging the Olympics.

Volgograd is a city of about 1 million people, about 690 kilometres north-east of Sochi, the host city of the Games, which is a major prestige project for Mr Putin.

It lies close to Russia's North Caucasus, a strip of mostly Muslim provinces plagued by near-daily violence in a long-running Islamist insurgency.

The latest travel advice from the Department of Foreign Affairs warns there is an ongoing risk of terrorism in Russia, particularly in the North Caucasus region.

ABC/wires