At least 10 people were killed when a bomb blast ripped through a trolleybus on Monday in the second deadly attack in the Russian city of Volgograd in two days, law enforcement authorities said.
The explosion came a day after a suicide bomber killed at least 17 people in the main railway station of the southern city 40 days before Russia is to hold the Winter Olympics in the Black Sea resort of Sochi.
State TV footage showed the twisted, gutted remains of the blue-and-white trolleybus, its roof blown off and debris strewn around the street. Federal investigators said the blast was believed to have been a "terrorist act".
The consecutive attacks underscored Russia's vulnerability to militants and will raise fears of a concerted campaign of violence before the Olympics, which start on Feb. 7 in Sochi, about 430 miles (690 km) southwest of Volgograd.
The Sochi Games are a major prestige project for President Vladimir Putin. Insurgents who want to carve an Islamic state out of southern Russia urged militants in a Web-posted video in July to use "maximum force" to prevent them from being held.
There was no immediate claim of responsibility for either of the attacks, which left dozens wounded in addition to the deaths. Authorities said 37 people were hospitalised after Sunday's attack and 23 were reported wounded on Monday.