* Trolleybus bombing follows Sunday's railway station attack
* Bombings underscore risks ahead of Sochi Games
* Moscow calls for international solidarity
* More attacks can be expected, says anti-terrorism expert
A bomb ripped a bus apart in Volgograd on Monday, killing 14 people in the second deadly attack blamed on suicide bombers in the southern Russian city in 24 hours and raising fears of Islamist attacks on the Winter Olympics.
President Vladimir Putin, who has staked his prestige on February's Sochi Games and dismissed threats from Chechen and other Islamist militants in the nearby North Caucasus, ordered tighter security nationwide after the morning rush-hour blast.
Investigators said they believed a male suicide bomber set off the blast, a day after a similar attack killed at least 17 in the main rail station of a city that serves as a gateway to the southern wedge of Russian territory bounded by the Black and Caspian Seas and the Caucasus mountains.
The blue and white trolleybus - powered by overhead electric cables - was reduced to a twisted, gutted carcass. Bodies were strewn across the street as Russians prepared to celebrate New Year, the biggest annual holiday.
Windows in nearby apartments were blown out by the blast, which Russia's foreign ministry condemned as part of a global "terrorist" campaign and welcomed a declaration of solidarity made on Sunday by the United Nations Security Council.
"For the second day, we are dying. It's a nightmare," a woman near the scene told Reuters, her voice trembling as she choked back tears. "What are we supposed to do, just walk now?"
"Identical" shrapnel to that in the rail station indicated that the two bombs were linked, investigators said.
"There was smoke and people were lying in the street," said Olga, who works nearby. "The driver was thrown a long way. She was alive and moaning ... Her hands and clothes were bloody."
There was no immediate claim of responsibility.
On Sunday, investigators initially described the station bomber as a woman from Dagestan, a hub of Islamist militancy on the Caspian, but they later said the attacker may have been a man. In October, a woman from the North Caucasus blew up and killed seven people on a bus in Volgograd.
It was unclear why the city, which will host soccer matches during the 2018 World Cup, has been hit. However, geography - being close to the restive regions - and its historical significance, may have contributed to its being targeted.
Volgograd has held a place in Russians' sense of national identity since, when known as Stalingrad, its Soviet defenders held off German invaders to turn the course of World War Two.
City authorities have revived the former name for special occasions as Stalin's image has been somewhat rehabilitated under Putin. He remains a hate figure, however, to Chechens, whose nation was deported en masse on the dictator's orders.
North Caucasus militants have also staged attacks in Moscow and other cities, the most recent in the capital being an airport suicide bombing three years ago that killed 37 people.
The violence raises fears of a concerted campaign before the Olympics, which start on Feb. 7 around Sochi, a resort on the Black Sea at the western end of the Caucasus range, 700 km (450 miles) southwest of Volgograd.
In an online video posted in July, the Chechen leader of insurgents who want to carve an Islamic state out of the swathe of mainly Muslim provinces south of Volgograd, urged militants to use "maximum force" to prevent the Games from going ahead.
"Terrorists in Volgograd aim to terrorise others around the world, making them stay away from the Sochi Olympics," said Dmitry Trenin, an analyst with the Moscow Carnegie Centre.
The International Olympic Committee expressed condolences and confidence in Russia's ability to secure the Games.
"Terrorism is a global phenomenon and no region is exempt, which is why security at the Games is a top priority for the IOC," said a spokeswoman at IOC headquarters in Lausanne.
In power since 2000, Putin secured the Games for Russia and has staked his personal reputation on a safe and successful Olympics. This month, he freed jailed opponents including oil tycoon Mikhail Khodorkovsky and the Pussy Riot punk band to remove causes for international criticism at the event.
Intended to showcase how Russia has changed since the collapse of Soviet communism in 1991, the Games have also been a focus for complaints in the West and among Russian liberals that Putin has stifled dissent and encouraged intolerance.
Putin was first elected after winning popularity for a war on Chechen rebels who had bid for independence after the break up of the Soviet Union. Attacks by Islamist militants whose insurgency is rooted in that war have clouded his 14 years in power and now present his biggest security challenge.
In a statement, the Foreign Ministry called on world powers to stand together against "terrorists" and named Chechen Islamist leader Doku Umarov as among those fomenting violence.
"We will continue our consistent fight against an insidious enemy that can only be stopped together," the ministry said.
Likening the Volgograd bombings to attacks in the United States, Syria, Afghanistan and elsewhere, it criticised those who seek to justify some such violence - an apparent dig at Western powers who have supported Syria's insurgents and who, at least in the 1990s, showed some sympathy for Chechen rebels.
Police said additional officers were being deployed to railway stations and airports nationwide after the bombing at the Volgograd rail station on Sunday, but the attacks raised questions about the effectiveness of security measures.
The police force in Volgograd, a city of a million people on the west bank of the river Volga, has been depleted as some 600 officers were redeployed to Sochi to tighten security around Olympic sites, a police officer told Reuters.
More attacks can be expected before the Olympics and cities in southern Russia where the Games are not being held are easier targets than Sochi, said Alexei Filatov, a prominent former member of Russia's elite anti-terrorism force, Alfa.
"The threat is greatest now because it is when terrorists can make the biggest impression," he said. "The security measures were beefed up long ago around Sochi, so terrorists will strike instead in these nearby cities like Volgograd."
The attacks also threaten to fuel ethnic tension, which has increased with an influx of migrant labourers from the impoverished Caucasus and Muslim Central Asian nations to cities around Russia, including Volgograd, in recent years.
"They need to be chased out of here. It has become a transit junction - there are all these non-Russians, both good and bad," said Olga, a saleswoman at a store near the mangled bus. "We've plenty bandits of our own. Why do we need others?"
Police were checking documents of people in Volgograd, with a focus on migrants, said Russian news agency Itar-Tass.
Christian and Muslim leaders appealed for calm. Russian Orthodox Patriarch Kirill said religion had "nothing to do with what is happening now in Volgograd".
Several dozen nationalists held a protest outside a chapel in the city on Monday and police detained over 20 of them.
"We are Russian; we must not be afraid," said Mikhail Yasin, a protester who lit a candle for the bombing victims.
"God is with us. We are in our own land and no terrorist can frighten us - nor can the police."