(CNN) -- Another deadly blast struck the southern Russian city of Volgograd on Monday, killing at least 10 people and further highlighting Russia's security challenges as it readies to host the Winter Olympics next month.
Monday's explosion hit a trolleybus during the morning rush hour, and comes one day after a blast at a train station in Volgograd killed 17 people and wounded at least 35.
Like Sunday's attack, the Monday incident was a terrorist act, Vladmiri Markin, a spokesman for the country's federal investigation agency, told the state-run news agency RIA Novosti reported
No one claimed responsibility for the explosions. But they come several months after the leader of a Chechen separatist group pledged violence to disrupt the 2014 Sochi Winter Olympics.
The runup to Sochi
Video footage from the scene Monday showed the twisted shell of the blue trolleybus, with debris spread around it. The impact of the blast blew out the roof of the bus as well as windows of several nearby houses.
Based on the footage, the blast appeared to have occurred in the back half of the bus. It was most likely caused by an explosive left inside, Russia's National Anti-Terrorism Committee told RIA Novosti.
The deadly explosions come less than six weeks ahead of the Winter Olympics in Sochi, located less than 1,000 kilometers (620 miles) from Volgograd.
The city, once called Stalingrad, is a major rail hub in the region, and each day thousands of passengers pass through the station, many on their way to Moscow.
In October, a bomber blew up a passenger bus in Volgograd, killing six people and wounding more than 30 others. Russian media reported that a female Islamist suicide bomber from the Russian region of Dagestan was responsible for that attack.
In July, Doku Umarov, the leader of the Chechen group Caucasus Emirate, released a video statement in which he vowed to unleash "maximum force" to disrupt the games.
The U.S. State Department considers the Caucasus Emirate a foreign terrorist group and has authorized a reward of up to $5 million for information leading to the location of Umarov.
The State Department said Umarov organized a suicide bombing outside the Chechen Interior Ministry in May 2009.
His group also claimed responsibility for last year's bombing of Domodedovo Airport in Moscow that killed 36 people, the 2010 bombings of the Moscow subway that killed 40 and the 2009 bombing of the high-speed Nevsky Express train in which 28 people died.