Russia's interior ministry said on Sunday that it was taking steps to increase security at all of the country's major train stations and airports.

"These measures involve a greater police presence and more detailed passenger checks," a ministry spokesman told Russia's Interfax news agency.

Investigators looking into what caused the attack on the main train station in the southern Russian city of Volgograd, which killed at least 16 people and injured many others on Sunday, say they now believe it was the work of a female suicide bomber.

No group immediately claimed responsibility, but there has been much speculation that Islamist militants active in Russia's North Caucasus region may have been behind the bombing. Several months ago their leader, Doku Umarov had called on his supporters to "do their utmost to derail" the Winter Olympic Games, which are to be hosted by the Russian city of Sochi in less than two months' time.

IOC confidence ahead of Sochi Games

While the attack has heightened concerns ahead of the Games, the International Olympic Committee has reiterated its confidence in the Russian authorities' ability to keep athletes and spectators safe.

"At the Olympics, security is the responsibility of the local authorities, and we have no doubt that the Russian authorities will be up to the task," the Associated Press cited an IOC statement as saying.

Russian planning for the Games includes some of the most extensive identity checks and other security measures ever to be implemented for an international sporting event. Among other things, anyone seeking to attend the Olympic events will be required to provide their passport details to purchase tickets and obtain a “security pass” needed to enter any of the venues.

Sunday's deadly bombing in Volgograd, which was formerly Stalingrad, has been met with wide international condemnation.

The UN Security Council condemned the attack "in the strongest terms." A statement issued by the current president of the Council, Gerard Araud, described the bombing as a "heinous act."

"There can be no excuse for such crimes. I hope that all those behind the attack will be brought swiftly to justice," the European Union's foreign policy coordinator, Catherine Ashton said.

Both Washington and the Western military alliance NATO expressed their solidarity with Russia following the attack.

A statement released by the US State Department said it stood "in solidarity with the Russian people against terrorism of any kind."

NATO and Russia "stand together in the fight against terrorism," said a statement released by NATO Secretary General Anders Fogh Rasmussen.

pfd/lw (AFP, AP)