The Black Sea city lies 690 kilometers southwest of Volgograd and in direct proximity to the violence ravaging North Caucasus regions such as Dagestan and Chechnya on a daily basis.

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 overtones for the Kremlin amid its efforts to use patriotism to mobilize support for President Vladimir Putin.

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 $50 billion for the event.

The Kremlin said Putin was "immediately" informed of the attack.

"He received detailed reports with all the preliminary information," Putin spokesman Dmitry Peskov said.

"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," the spokesman said.

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.

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