Two more people wounded by suicide bombings in the Russian city of Volgograd have died, increasing the death toll in the attacks to 33, Interfax news agency reported on Tuesday, citing the country's emergencies ministry.
The city's railway station was bombed on Sunday and a bus was ripped apart on Monday, raising fears of Islamist attacks on the upcoming Winter Olympics in Sochi, a resort on the Black Sea 700 km (450 miles) southwest. The Olympics will start on February 7.
Citing the ministry's regional spokesman, Interfax said that the number killed in the main rail station on Sunday rose by one to 18, while the bus bombing death toll rose by one to 15.
"At night in the Volgograd hospital, one victim of the railway explosion died, and the number of victims has increased to 18," said Dmitry Ulanov, a regional spokesman of the emergencies ministry, Interfax reported.
A Reuters journalist saw the blue-and-white trolleybus reduced to a twisted, gutted carcass, its roof blown off and bodies and debris strewn across the street. Federal investigators called the blast a"terrorist act".
"For the second day, we are dying. It's a nightmare," a woman near the scene said, her voice trembling as she choked back tears. "What are we supposed to do, just walk now?"
There have been no claims of responsibility for the attacks.
However, in a video posted on the web in July, the leader of insurgents who want to carve an Islamic state out of the North Caucasus, a string of Muslim provinces south of Volgograd, urged militants to use "maximum force" to prevent the games from being held.
"Terrorists in Volgograd aim to terrorize others around the world, making them stay away from the Sochi Olympics," said Dmitry Trenin, an analyst with the Moscow Carnegie Centre.
A female suicide bomber from the North Caucasus killed seven people on a bus in Volgograd in October.
Biggest security challenge
In power since 2000, Vladimir Putin secured the Games for Russia and has staked his reputation on a safe and successful Olympics, even freeing jailed opponents including oil tycoon Mikhail Khodorkovsky and the Pussy Riot punk band to remove a cause for international criticism at the event.
Putin was first elected after winning popularity for a war against Chechen rebels, but attacks by Islamist militants whose insurgency is rooted in the war have clouded his 14 years in power and now confront him with his biggest security challenge.
Putin ordered law enforcement agencies to take measures to enhance security after Sunday's attack and police said additional officers were being deployed to railway stations and airports nationwide, but the attacks raised questions about the effectiveness of security measures.
The police force in Volgograd has been depleted in recent months as some 600 officers were redeployed to Sochi to tighten security around Olympic sites, a local 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, deputy head of the veterans' association of the elite Alfa anti-terrorism unit.
"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."
Monday's blast was so powerful it blew out the third-storey windows of a nearby apartment building, and witnesses said passengers were flung from the bus by the blast.
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"We ran outside. There was smoke and people were lying in the street. The driver was thrown far. She was alive and moaning ... Her hands and clothes were bloody," said Olga, a clerk in a shop near the scene of the blast.
A city of about 1 million, Volgogradis one of the venues for the 2018 soccer World Cup, another high-profile sports event Putin has helped Russia win the right to stage, and which will bring thousands of foreign fans to cities around Russia.
Sunday's attack was the deadliest to strike Russia's heartland since January 2011, when a male suicide bomber from the North Caucasus killed 37 people in the arrivals hall of a busy Moscow airport.
Investigators initially said a woman set off the bomb that filled the front of the railway station building with a massive orange fireball on Sunday, but later said the bomber could have been a man.
A spokeswoman for the International Olympic Committee in Lausanne, Switzerland, said: "Our condolences go to all those affected by today's bombing in Volgograd. Unfortunately, terrorism is a global phenomenon and no region is exempt, which is why security at the Games is a top priority for the IOC. 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."