The most deadly incident happened in the eastern Siberian city of Chita, where in the late afternoon a blast rocked a cafe in Karl Marx street, destroying it completely.

The death toll was initially reported at nine but rapidly climbed to 17 as rescuers sifted through the ruins for bodies. Another 17 people were injured, seven of them in serious condition, the emergencies ministry in Moscow said.

Some 160 rescuers, medical personnel and firemen were at the scene, officials added. Prosecutors opened a criminal inquiry for the flouting of fire safety rules leading to people's deaths.

In another part of Russia, two 14-year-old girls and two 15-year-old boys were killed in a boiler room explosion, believed to have been caused by a gas leak at their school in the southern republic of Dagestan.

The boiler is thought to have blown up due to a "faulty operation," causing it to collapse onto adjoining toilets where five schoolchildren were at the time of the blast, officials said. One girl was injured and is in emergency care in hospital.

"At the moment of the blast, the children who died were in the toilet, which is located between the boiler room and the school building. The school was not destroyed by the blast," an emergencies ministry official told the Interfax news agency.

The school where the blast occurred is in a village around 30 kilometres (20 miles) south of the republic's capital.

"Everyone was very frightened when we heard the explosion and we rushed out of our classrooms. One of my friends was killed," one young schoolboy said on NTV television.