An explosion at a train station in the southern Russian city of Volgograd has killed at least 13 people, reports say.
A female suicide bomber was thought to be responsible for the blast, Russia's anti-terrorism committee 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.
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.
Security would be stepped up at train stations and airports, said a federal police spokesman.
No group has yet claimed responsibility for the blast, which local officials said claimed at least 18 lives and left dozens more injured.
"Initial indications are that the blast was set off by a female suicide bomber," said the National Anti-Terror Committee said in a statement.
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, said Investigative Committee spokesman Vladimir Markin.