Moscow - At least 18 people were killed and dozens injured on Sunday when a female suicide bomber blew herself up in a train station in the southern Russian city of Volgograd ahead of Olympic Games in nearby Sochi.
Regional officials said the woman set off her charge after being stopped at the metal detectors at the entrance to the city's main train station while it was packed with people traveling to celebrate the New Year.
"It was a very powerful blast," train station store attendant Valentina Petrichenko told the Vesti 24 news channel.
"Some people started running and others were thrown back by the wave of the blast," she said. "It was very scary."
State television footage showed windows blown out across the top two floors of the grey stone building and smoke billowing out its vast front doors.
"Initial indications are that the blast was set off by a female suicide bomber," the National Anti-Terror Committee said in a statement.
Russia's Investigative Committee spokesman Vladimir Markin said officials had launched an inquiry into a suspected "act of terror".
A regional government spokesman told the state-run RIA Novosti news agency that at least 18 people were killed and more than 40 injured in the attack.
A police official told Interfax investigators had come across the severed head of a woman "that is believed to have belonged to the suicide bomber."
Volgograd Mayor Irina Guseva said that "the situation in the city was difficult" because one child was killed and several others seriously injured in the blast.
"But we will not allow panic to grip this city," she told Vesti 24 television.Olympic security fears
The city of Volgograd - known as Stalingrad in the Soviet era - was already attacked in October by a female suicide bomber with links to Islamists fighting federal forces in the nearby volatile North Caucasus.
The October 21 strike killed six people aboard a crowded bus and immediately raised security fears ahead of the 7 - 23 February Winter Games in Sochi.
The Black Sea city lies 690km southwest of Volgograd and in direct proximity to the violence ravaging daily in North Caucasus regions such as Dagestan and Chechnya.
Militants are seeking to impose an Islamist state throughout Russia's North Caucasus. Their leader Doku Umarov has ordered his foot soldiers to target civilians outside the region and disrupt the Olympic Games.
The Sochi Games' success carries heavy political overtones for the Kremlin amid its efforts to use patriotism to mobilise support for President Vladimir Putin's 14-year rule.
Putin staked his personal reputation on the Games' success by lobbying for Sochi's candidacy before the International Olympic Committee and then spending more than $50bn for the event.
The Kremlin said Putin was "immediately" informed of the attack.
"He received detailed reports with all the preliminary information," Putin spokesperson Dmitry Peskov told Vesti 24.
"The president is also receiving reports as the events develop and new information comes in - first of all, this concerns the number of people injured and killed," said the spokesperson.
Peskov said Putin also issued orders to the emergency and health ministries to send the most gravely injured victims for necessary treatment to Moscow.
Russia's interior ministry said separately that it was immediately stepping up security at all the nation's main train stations and airports.
"These measures involve a greater police presence and more detailed passenger checks," an interior ministry spokesman told the Interfax news agency.'Black widows'
Russian authorities have repeatedly vowed to take the highest security precautions in Sochi. There have been few indications to date of foreign sports fans cancelling their attendance out of security fears.
Female suicide bombers are often referred to in Russia as "black widows" - women who seek to avenge the deaths of their family members in North Caucasus fighting by targeting Russian civilians.
Female suicide bombers set off blasts at two Moscow metro stations in March 2010 that killed more than 35 people.
So-called black widows were also responsible for killing more than 90 people when they took down two passenger jets that took off from a Moscow airport within minutes of each other in 2004.