An attack on a train station in Russia's southern city of Volgograd has killed 15 people, officials say.
A female suicide bomber was thought to be responsible for the blast, Russia's top investigative agency said.
A suspected female suicide bomber killed at least six people when she attacked a bus in the city in October.
Moscow is concerned militant groups could be ramping up violence in the run up to the the 2014 winter Olympic Games in the city of Sochi in six weeks.
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.
The bomb contained 10 kg (22 pounds) of TNT, was rigged with shrapnel and was detonated near the metal detectors at the station entrance, said a spokesman for the Investigative Committee.
"When the suicide bomber saw a policeman near a metal detector, she became nervous and set off her explosive device," said the spokesman, Vladimir Markin.
He said the security presence had prevented a much higher death toll at the station, which was packed at the time of the blast as several trains were delayed.
No group has yet claimed responsibility for the blast, which Mr Markin said injured 34 people - eight critically - including a nine-year-old girl whose mother was killed in the attack.
The Interfax news agency said the suspected bomber's head had been found at the site, which would allow her swift identification.
RIA Novosti news agency said security sources were naming the attacker as Oksana Aslanova. She has reportedly been married twice to militants and is also suspected of being a friend of Naida Asiyalova, the suicide bomber who targeted the Volgograd bus in October.
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.