(CNN) -- IOC president Thomas Bach has condemned the "despicable attack" on Volgograd which has raised fresh fears about security at the forthcoming Winter Olympics in Sochi.
At least 31 people have been killed in two separate explosions Sunday and Monday within the space of 24 hours, with the main railway station and a trolleybus targeted.
Volgograd is 690km from the Black Sea resort where the Winter Games is due to begin on February 7, but it is a public transport hub where people traveling to Sochi by rail would likely use.
Bach, set to take charge of his first Games since being elected to the top job at the International Olympic Committee in October, issued a strongly-worded statement Monday in reaction to the bombings.
"This is a despicable attack on innocent people and the entire Olympic Movement joins me in utterly condemning this cowardly act. Our thoughts are with the loved ones of the victims," Bach said.
"I have personally written to the President of the Russian Federation, Vladimir Putin, to express our condolences to the Russian people and our confidence in the Russian authorities to deliver safe and secure Games in Sochi.
"I am certain that everything will be done to ensure the security of the athletes and all the participants of the Olympic Games.
"Sadly terrorism is a global disease but it must never be allowed to triumph. The Olympic Games are about bringing people from all backgrounds and beliefs together to overcome our differences in a peaceful way.
"The many declarations of support and solidarity from the international community make me confident that this message of tolerance will also be delivered by the Olympic Winter Games in Sochi."
Russian authorities have reacted to the attacks by stepping up security at major train stations and airports.
No group has to date claimed responsibility for the attacks, but suspicion has fallen on Chechen separatist groups.
The leader of one of the groups, Doku Umarov, has ordered rebels to target civilians outside of Russia's North Caucasus and to disrupt the Winter Games.
Sochi organizers told CNN that "Games security is being managed by the authorities in a highly rigorous way.
"It will remain of paramount importance with an unprecedented level of planning and measures put in place over the last seven years.
"We are confident the Games in Sochi are going to be safe and comfortable for all."
The organizing committee of the 2018 Football World Cup in Russia said it was "constantly reviewing" its security plans and said it was "deeply saddened" by the incidents in Volgograd, which is set to host matches at its 45,000 capacity Central Stadium.