At least 14 people have been killed and 28 wounded in a suicide bomb attack on a packed trolley-bus in Volgograd, according to a Russian Health Ministry spokesman.

The city was on high alert on Monday following the latest attack, which came a day after a suicide bombing at a train station there killed 17 people.

"The explosives were detonated by a male suicide bomber, fragments of whose body have been found and taken for genetic analysis to establish his identity," Vladimir Markin, spokesman for Russia's federal Investigative Committee, said.

The force of Monday's blast destroyed the number 15A trolley-bus, which was packed with early morning commuters at about 8:30am local time (0430GMT).

Markin said the suicide bomber set off four kilograms of explosives, and noted that the explosives were identical to those used in Sunday's train-station bombing.

"This confirms the theory that the two attacks are linked. It is possible that they were prepared in the same place," he said.

Volgograd, a city of a million people on the west bank of the River Volga, serves as a gateway to the southern wedge of Russian territory bounded by the Black and Caspian Seas and the Caucasus mountains.

Russia's Foreign Ministry called for international solidarity in the fight against "terrorism" on Monday, though it didn't place direct blame on any group for the bombings.

"We will not retreat and will continue our consistent fight against an insidious enemy that can only be defeated together," the ministry said in a statement.

Russian investigators have opened a criminal probe into a suspected act of terror as well as the illegal carrying of weapons, Markin told Interfax.

Russian President Vladimir Putin has sent Alexander Bordnikov, director of Federal Security Service, to Volgograd while the Interior Ministry raised the number of its staff in the city.

The ministry was also expected to send a plane to Volgograd for transporting the wounded in Monday's attack to Moscow for medical treatment, according to Interfax.

The attack was the third on the city in the past three months.

On October 21, a female suicide bomber blew herself up on a bus, killing six people.

"The people are angry. They feel let down by the social services, by the police, by the security forces. They feel not enough has been done to protect them, to protect their families," Al Jazeera's Peter Sharp said from Volgograd on Monday.

"About 400 local people gathered to protest about what's happened here over the last 24 hours, and the police cleared them off saying their presence altogether, en masse, was just another target for the people who are carrying out these bombings," our correspondent said.

Threat to Winter Olympics

Russian officials said security would be stepped up in Moscow before New Year, the biggest holiday of the year in the country, which is traditionally marked by mass outdoor gatherings.

Reports said Russia's second city of Saint Petersburg had already cancelled its planned New Year fireworks display.

The blasts have also raised safety concerns in the region in the run-up to the Winter Olympics 2014 due to take place in February in Sochi, about 650km southwest of Volgograd.

There was no immediate claim of responsibility for either of the Volgograd attacks, which came several months after Chechen rebel leader Doku Umarov called for new attacks against civilian targets in Russia, including Sochi Games.

In the past, armed groups have carried out attacks in cities beyond the borders of the North Caucasus.

Rebels have said they want to carve out a Muslim state in the North Caucasus. Their armed campaign is rooted in two post-Soviet wars in Chechnya, one of the region's provinces.

Putin, who was first elected president in 2000 after launching the second war in Chechnya, which drove Chechen separatists from power, has staked his reputation on a safe and successful Winter Games.

Al Jazeera a