A bomb blast ripped apart a trolleybus in the southern Russian city of Volgograd Monday, killing 14 people in the second suicide bombing in the city in as many days.
Monday's blast came just hours after a suicide bomber detonated explosives at the security entrance of the city's main train station in a Sunday attack that left at least 17 dead.
A spokesman for Russia's main investigative agency says the bomb involved in Monday's explosion was similar to the one used in Sunday's attack, confirming suspicions that they may be linked.
Authorities say nearly 30 people were wounded in Monday's powerful blast which destroyed the bus and blew out the windows of nearby buildings.
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.
There was no immediate claim of responsibility for either explosion. 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.