Russian officials say three more people have died as a result of the two suicide bombings in the southern city of Volgograd Sunday and Monday, bringing the death toll to 34 with dozens more injured.
No one has yet claimed responsibility for the attacks.
Russian authorities have increased security 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.
The Kremlin said Monday Russian President Vladimir Putin has ordered the country's counterterrorism agency to step up security in Volgograd and elsewhere across the country.
Russia's foreign ministry compared the attacks to similar acts of terrorism in the United States, Iraq and Nigeria. They called for international solidarity in countering terrorism and the "ideology of violence that feeds it as well as extremism".
Anatoly Ermolin, a veteran of Russia's special security forces, told VOA's Russian Service that what happened in Volgograd was a planned terrorist act and carried out professionally. He says it is a "layered attack" which may not be over, adding that the goal was to have the whole country talking about it in the lead up to New Years celebrations.
U.S. National Security Council spokesperson Caitlin Hayden condemned the attacks in Russia, saying the United States stands with the Russian people against terrorism. Hayden's statement says the U.S. government has offered its full support to the Russian government in security preparations for the upcoming Olympic Games.
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 at the Games.
Russian Olympic Committee chief Alexander Zhukov says no additional security measures will be taken in Sochi in the light of the attacks in Volgograd, adding that "everything necessary has been done."
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.
Authorities initially said Sunday's blast was set off by a female suicide bomber from Dagestan - a republic in the nearby volatile North Caucasus. But authorities later said they believe the attacker was a man.
Some information for this report was provided by AP, AFP and Reuters.