Russian President Vladimir Putin has vowed to fight terrorists until their "total destruction," following the two suicide bombings in the southern city of Volgograd on Sunday and Monday that left 34 people dead.
In his annual New Years address, Putin said that at the country's most trying times, Russia has always been united and consolidated.
Russian authorities have deployed more than 5,000 security force personnel in and around Volgograd in the wake of the bombings.
In Monday's attack, a bomb blast ripped apart a trolleybus, killing 16 people and injuring 30 others. A day earlier, a suicide bomber detonated explosives at the security entrance of the city's main train station in an attack that left 18 dead.
A spokesman for Russia's main investigative agency said the bomb in Monday's explosion was similar to the one used in Sunday's attack, confirming suspicions that they may be linked.
Russia's foreign ministry compared the attacks to similar acts of terrorism in the United States, Iraq and Nigeria. It called for international solidarity in countering terrorism.
The attacks came just weeks before the Winter Olympics open in Sochi, about 650 kilometers southwest of Volgograd. Islamist militants had threatened to attack civilians and disrupt the Winter Games.
The International Olympic Committee expressed its condolences over the bombings, but says it is confident of Russia's ability to provide security.
Russia has introduced some of the most stringent security at any international sporting event, including a limited access security cordon around the entire city of Sochi and requiring spectators to have accreditation documents which include passport details and contact information.