Russian authorities have increased security in and around the southern city of Volgograd after two suicide bomb attacks killed 31 people and left dozens more injured over the past two days.
In Monday's attack, a bomb blast ripped apart a trolleybus killing 14 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 at least 17 dead.
A spokesman for Russia's main investigative agency says the bomb in Monday's explosion was similar to the one used in Sunday's attack, confirming suspicions that they may be linked.
There was no immediate claim of responsibility for either blast.
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 say Sunday's blast, which wounded dozens of people, was set off by a female suicide bomber from Dagestan - a republic in the nearby volatile North Caucasus.
An attack in Volgograd by a female suicide bomber on October 21 killed five people and wounded 30. Investigators also identified her as coming from Dagestan.
Dagestan is one of the centers of an ongoing Islamist insurgency in the North Caucasus.
In early July, the leader of the Islamist insurgency in the North Caucasus, Doku Umarov, declared an end to a moratorium on attacks on Russian civilian targets that he had announced the previous year.