A Russian court on Friday convicted four men for helping orchestrate twin suicide blasts in 2013 that killed 34 people, handing down sentences of up to 19 years in detention.

The attacks against the main railway station in the southern city of Volgograd and a trolleybus took place on two consecutive days in late December, sparking security fears only two months before Russia hosted the Winter Olympic Games.

Russia beefed up security and mourned its dead on Tuesday as the toll from jarring successive-day suicide strikes in the run-up to the Sochi Winter Olympic Games rose to 34.

The southern city of Volgograd planned to bury the first victims of Sunday's bombing of the heartland hub's main railway station that killed 18 people and which officials have provisionally blamed on a young woman from the restive North Caucasus region of Dagestan.

A blast that tore through an electric bus in the southern Russian city of Volgograd during Monday's morning rush hour, killing 14, was probably carried out by suicide bombers from the same organization behind a railway explosion a day earlier, officials said.

Together more than 30 people were killed in the explosions, putting the city of one million on edge and highlighting the terrorist threat Russia is facing as it prepares to host February's Winter Games in Sochi, President Vladimir Putin's pet project. While terrorists may find it hard to get to the tightly guarded Olympic facilities, the bombings have shown they can hit civilian targets elsewhere inRussia with shocking ease.

Fourteen people were killed and 34 injured Sunday when a female suicide bomber blew herself up in a train station in the southern Russian city of Volgograd, investigators said, revising down an earlier toll.

A Volgograd government spokesman had earlier told the state-run RIA Novosti news agency that at least 18 people were killed and more than 40 injured in the attack.

A suspected female suicide bomber blew herself up on a bus packed with students in southern Russia on Monday, killing six people and raising security fears less than four months before the Winter Olympics.

The attack in the Volga River city of Volgograd, which also injured more than 30 people, was the deadliest outside the volatile North Caucasus in the past three years.

The city of Volgograd was renamed Stalingrad for a day Saturday as Russia marked the 70-year anniversary of a brutal battle in which the Red Army defeated Nazi forces and changed the course of World War II.

Commuter buses emblazoned with pictures of the feared Soviet dictator ran across the southern city as patriotic Russians remembered what many view as the Soviet people's greatest achievement.