The blast, in the town of Shakhty in the southern Rostov region, destroyed several apartments on the top two floors of the nine-storey building, the emergencies ministry said.
The blast came two weeks after a gas explosion tore through a Russian apartment block in the Urals city of Magnitogorsk on New Year's Eve, leaving 39 people dead. Chernyavskaya said the building in Shakhty, 1,000 kilometres (620 miles) south of Moscow, was in danger of collapse and officials planned to prop it up.
Authorities said the rescue operation in Shakhty would take at least 24 hours. A man and a two-year-old girl were hospitalised and their condition was "satisfactory," said Chernyavskaya.
More than 40 people were evacuated to a nearby school.
Earlier in Magnitogorsk, investigators ruled out foul play, saying the accident came as a result of a gas explosion.
Gas explosions are relatively common in Russia, where much of the infrastructure dates back to the Soviet era and safety requirements are often ignored.
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