More than 25 people were killed in two major blasts in Russia and counter-terrorism experts warned of more attacks as the couuntry is set to host the Winter Olympics in 40 days.

At least 10 people were killed when an explosion ripped through a trolleybus in Volgograd in two days, the Interfax news agency reported, citing law enforcement officials.

The explosion came a day after a suicide bomber killed at least 17 people in the main railway station of the southern Russian city.

Russia is to hold the Winter Olympics in the Black Sea resort city of Sochi.

Authorities said the suicided attacker detonated a shrapnel-filled bomb in front of a metal detector just inside the main entrance of the station in Volgograd, a busy hub north of the violence-plagued North Caucasus region on Russia's southern fringe.

Islamist militants in the North Caucasus have carried out a long string of attacks since President Vladimir Putin came to power in 2000. They now confront him with his biggest security challenge, threatening to disrupt the Olympics that start in the Black Sea resort of Sochi in 40 days.

Footage shown on TV captured the moment of the blast, as a massive orange fireball filled the hall of the stately, colonnaded station and clouds of grey smoke poured out of shattered windows.

The station - a Stalinesque architectural monument with a clocktower and spire topped by a Soviet-style star - was busier than usual, with people travelling home for the New Year, one of the main holidays in Russia.

"People were lying on the ground, screaming and calling for help," a witness, Alexander Koblyakov, told Rossiya-24 TV. "I helped carry out a police officer whose head and face were covered in blood. He couldn't speak."

The city once bore the name Stalingrad in honour of Soviet dictator Josef Stalin, a figure held in opprobrium by many in the North Caucasus.

In the 1940s, Stalin ordered the deportation of tens of thousands of people from the region, including Chechens, to Central Asia on suspicion of harbouring sympathies for Nazi Germany. Many thousands died in exile and transport.

The federal Investigative Committee and other officials initially said a female suicide bomber had blown herself up after a police officer started to approach her near the metal detector because she looked suspicious.

A Russian website with ties to security agencies, Life News, posted a picture of what it said was the suspect's head.

It said authorities had identified her as a resident of Dagestan, the province adjacent to Chechnya and now the centre of a long-running Islamist insurgency, and the widow of two militants who were both killed by Russian security forces.

Investigative Committee spokesman Vladimir Markin later said a man could have set off the blast, Russian news agencies reported.

Interfax cited law enforcement sources as saying authorities believed the attacker was a man who brought a bomb into the station with a rucksack. Some bombs carried by female suicide bombers have been set off remotely by male accomplices.

So-called 'black widows', seeking to avenge fallen husbands, were involved in a deadly Moscow theatre siege in 2002 and have been behind several bombings including twin suicide attacks that killed 40 on the Moscow subway in 2010.

"We can expect more such attacks," said Alexei Filatov, deputy head of the veterans' association of the elite Alfa anti-terrorism unit.

"The threat is greatest now because it is when terrorists can make the biggest impression," he told Reuters. "The security measures were beefed up long ago around Sochi, so terrorists will strike instead in these nearby cities like Volgograd."

The insurgency is rooted in two post-Soviet separatist wars in Chechnya, the second of which was launched by Putin as Prime Minister and succeeded in driving separatists from power.

Markin said 16 people were killed in the attack, including two who died in hospital. A regional government official also put the toll at 16 and said that did not include the attacker.

TIGHTER SECURITY

Putin ordered law enforcement agencies to take all necessary precautions to ensure security, his spokesman said. Police said security would be tightened at stations and airports, with more officers on duty and stricter passenger checks.

The US State Department issued a statement saying that the United States "condemns in the strongest terms today's terrorist attack in Volgograd."

"We send our sincere condolences to the families of the victims and stand in solidarity with the Russian people against terrorism of any kind," it said. - Reuters