An explosion at a train station in the southern Russian city of Volgograd has killed at least 14 people, reports say.
A female suicide bomber was thought to be responsible for the blast, Russia's anti-terrorism committee said.
In October, a suspected female suicide bomber killed at least six people when she attacked a bus in the city.
In June, Doku Umarov, one of the leaders of the Islamist
insurgency in the Russian Caucasus republics, called on
his supporters to use "maximum force" to disrupt the
"satanic" winter Olympics in Sochi.
It is too early to say whether the attack in Volgograd was
by one of his supporters.
But it shows that - despite the metal detectors at railway
stations, airports and shopping centres in Russia,
bombers are still able to kill and wreak havoc.
It also shows that the attacks will not have to be on Sochi
itself to attract attention.
Footage from a CCTV camera showed the moment of
Moscow is concerned militant groups could be ramping up violence in the run up to the 2014 winter Olympic Games in the city of Sochi in six weeks.
An Islamist insurgency in the North Caucasus region has led to many attacks there in recent years. Insurgents have also attacked big Russian towns.
Volgograd lies about 900km (560 miles) south of Moscow, 650km north of the North Caucasus and 700km north-east of Sochi.
Russian President Vladimir Putin has ordered law enforcement agencies to take "all necessary security measures" in the bomb's aftermath, said a Kremlin spokesman.
Mr Putin has ordered the most gravely injured victims to be flown to Moscow for treatment.
Security would be stepped up at train stations and airports, said a federal police spokesman.
'Act of terrorism'
Sunday's explosion rocked Volgograd-1 station at around 12:45 (08:45 GMT) at a time when millions of Russians are travelling to celebrate the New Year.
No group has yet said it carried out the bombing, in which dozens of people were injured.
"Initial indications are that the blast was set off by a female suicide bomber," said the National Anti-Terror Committee in a statement.
The blast blew out many windows and sent debris down the station steps
Formerly known as Stalingrad, Volgograd has a million residents and is described as a gateway to Russia
Sources say the bomb was detonated near metal detectors at the station entrance.
Vladimir Markin, the spokesman for the nation's top investigative agency, the Investigative Committee, claimed that security measures within the station had prevented the bomber hitting a far larger number of people.
"When the suicide bomber saw a policeman near a metal detector, she became nervous and set off her explosive device," which contained some 10kg (22lb) of TNT and was packed with shrapnel, Mr Markin said in a statement quoted by the Associated Press.
That prevented her going further into the station, which was full of people at a time when several trains were delayed, he said.
A nearby security camera facing the station caught the moment of the blast, showing a bright orange flash behind the station's main doors.
The explosion shattered windows and sent debris and plumes of smoke from the station entrance.
Ambulances rushed the injured to hospital, while motionless bodies were laid out in the station forecourt.
The incident was being treated as an act of terrorism, Mr Markin said.
In July, Chechen insurgent leader Doku Umarov posted an online video urging militants to use "maximum force" to prevent the Games from going ahead.
On Friday, a car bomb killed three people in the southern Russian city of Pyatigorsk.