MOSCOW (AP) — A bomb blast tore through a trolleybus in the city of Volgograd on Monday morning, killing at least 10 people a day after a suicide bombing that killed at least 17 at the city's main railway station.

The explosions put the city on edge and highlighted the terrorist threat that Russia is facing as it prepares to host the Winter Olympics in February. Volgograd is about 650 kilometers (400 miles) northeast of Sochi, where the Olympics are to be held.

The National Anti-Terrorism Committee said the bus explosion came from a bomb that most likely had been placed in the vehicle's passenger area, but there were no further details. The Emergencies Ministry said at least 10 people were killed and news reports said anywhere from 15 to two dozen people were hospitalized.

The explosion ripped away much of the bus's exterior and broke windows in nearby buildings.

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

Suicide bombings have rocked Russia for years, but most have been in the North Caucasus region, the center of an insurgency seeking an Islamist state in the region. But Volgograd, formerly known as Stalingrad, has been struck three times in two months — suggesting militants may be using the transportation hub as a renewed way of showing their reach outside their restive region.

A suicide bus bombing in Volgograd in October killed six people. On Friday, three people were killed when an explosives-rigged car blew up in the city of Pyatigorsk, the center of a federal administrative district created to oversee Kremlin efforts to stabilize the North Caucasus region.

In the railroad station blast, the bomber detonated explosives in front of a metal detector just beyond the station's main entrance when a police sergeant became suspicious and rushed forward to check ID, officials said. The officer was killed by the blast, and several other policemen were wounded.

Following Sunday's explosion, the Interior Ministry ordered police to beef up patrols at railway stations and other transport facilities across Russia.

Russia in past years has seen a series of terror attacks on buses, trains and airplanes, some carried out by suicide bombers.

Twin bombings on the Moscow subway in March 2010 by female suicide bombers killed 40 people and wounded more than 120. In January 2011, a male suicide bomber struck Moscow's Domodedovo Airport, killing 37 people and injuring more than 180.

Umarov, who had claimed responsibility for the 2010 and 2011 bombings, ordered a halt to attacks on civilian targets during the mass street protests against President Vladimir Putin in the winter of 2011-12. He reversed that order in July, urging his men to "do their utmost to derail" the Sochi Olympics which he described as "satanic dances on the bones of our ancestors."

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Associated Press writer Jim Heintz in Moscow contributed to this report.