By Jethro Mullen, CNN
updated 7:43 AM EST, Mon December 30, 2013
(CNN) -- Another deadly blast has struck the southern Russian city of Volgograd, killing at least 14 people and further highlighting Russia's security challenges as it readies to host the Winter Olympics in less than six weeks.
An explosion hit a trolleybus near a busy market during the morning rush hour Monday, a day after a blast at Volgograd's main train station killed 17 people and wounded at least 35 others.
Like Sunday's attack, the blast Monday was a terrorist act, Vladmir Markin, a spokesman for the country's federal investigation agency, told the state-run news agency RIA Novosti.
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