At least 15 people were killed Monday and 23 others hurt in a suspected suicide bombing on a trolleybus in the Russian city of Volgograd, officials say.
The blast comes a day after 17 people died in another suicide attack at the central train station in the city.
The BBC reported that security has been tightened at railway stations and airports across Russia.
Moscow is concerned terror groups could be ramping up violence in the run-up to the 2014 winter Olympic Games in the city of Sochi, which is close to Russia's volatile north Caucasus region.
Volgograd lies about 900km (560 miles) south of Moscow, 650km north of the North Caucasus and 700km north-east of Sochi.
A spokesman for Russia's Investigative Committee said both explosions were now being treated as acts of terrorism.
The latest explosion took place near a busy market in the city's Dzerzhinsky district during Monday morning rush hour. A one-year-old child is reported to have been among those injured.
On Sunday, a suspected female suicide bomber killed 18 in an attack at the Volgograd-1 train station.
On December 27, a car bomb killed three in the southern city of Pyatigorsk
On October 21, a female suicide bomber killed six in an attack on a bus in Volgograd.