MOSCOW (AFP) - 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 33.
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 second suicide attack on a trolleybus packed with students on Monday that investigators pinned on a male assailant has now claimed 15 lives.
The blasts are Russia's deadliest since a suicide bombing at Moscow's Domodedovo airport that was claimed by Islamic insurgents from the North Caucasus killed 37 people in January 2011.
The latests blasts have laid bare the unchecked threat posed by guerrillas who have vowed to target civilians in a bid to undermine President Vladimir Putin as he welcomes world guests to Sochi for the Game's opening ceremony on February 7.
Putin has not spoken publically about the tragedies but is expected to address the nation during a midnight New Year address that will be watched closely for signs of how he intends to act next.
The powerful Russian leader issued a string of directives on Monday ordering security stepped up at public transit points across the nation and extra police deployed on the streets of Volgograd.
The city of one million - known as Stalingrad in the Soviet era and which is 690 kilometres northeast of Sochi - saw more than 5,000 law enforcement agents check traffic and inspect public transport on Tuesday morning.
"The operational headquarters of the Volgograd region has decided to call up extra reserves and enrol the maximum number of police and interior ministry soldiers possible," Interfax quoted regional security spokesman Andrei Pilipchuk as saying.
The nation's main Channel 1 television station led the news with images of helmeted soldiers with automatic rifles bursting into a Volgograd cafe and checking documents of dark-haired men who looked like they may have come from the mostly-Muslim North Caucasus.
Pilipchuk told the station that officers had arrested 87 people "who resisted police or did not have (their identity) documents".
Russia is already preparing to impose a "limited access" security cordon around Sochi from January 7 that will check all traffic and ban all non-resident cars from a wide area around the city.
Russian investigators have opened a criminal probe into a suspected act of terror as well as the illegal carrying of weapons.
The chief spokesman for the Investigative Committee - Russia's equivalent to the US FBI - said the signature of the two bombings suggest that they were plotted by the same group.
The identical makeup of the explosives "confirms the theory that the two attacks are linked. It is possible that they were prepared in the same place," Investigative Committee spokesman Vladimir Marking said.
No one has claimed responsiblity for either attack.
But North Caucasus guerrilla chief Doku Umarov vowed in July that his fighters would use "any means possible" to keep Putin from staging the Sochi Games.
The feared fighter - pronounced dead by Russia on repeated occasions - is seeking to carve out an Islamist republic across the North Caucasus and has claimed the region around Sochi as part of his self-proclaimed ancestral land.
"This looks like the North Caucasus underground. They promised to stage acts in big Russian cities ahead of the Olympics. It seems they are fulfilling their promise," military affairs commentator Pavel Felgenhauer told AFP.
The head of the International Olympic Committee said he was confident that security would be all-encompassing in Sochi and gave no indication that the Games' staging was in doubt.
"The Olympic Games are about bringing people from all backgrounds and beliefs together to overcome our differences in a peaceful way," Thomas Bach said.
The United States meanwhile offered to assist policing Sochi and called for closer security cooperation that had been recently lacking as relations between powers plumbed post-Cold War lows.
US National Security Council spokeswoman Caitlin Hayden said Washington "would welcome the opportunity for closer cooperation for the safety of the athletes, spectators, and other participants".
UN leader Ban Ki Moon and the UN Security Council both pronounced themselves "outraged".
Putin assured Ban in a telephone conversation "that Russia would work within the bounds of international law to bring the perpetrators to justice", UN spokesman Martin Nesirky said.
German Chancellor Angela Merkel also condemned the attacks while EU foreign policy chief Catherine Ashton said she was "shocked".
"I hope that those behind these crimes will be brought to justice," Ashton said in a statement.