Local officials said residents could be trapped under the rubble and that around 100 people have been made homeless, but did not confirm reports that two people may have died.
"According to preliminary data, a gas explosion occurred Dec 20 at around 12 pm in a multi-storey apartment building," the investigative committee said in a statement.
Eight people, including two children aged five and seven, are in hospital but the committee said the children's lives are not in danger, it said.
Regional emergency services said a section of the apartment building had partially collapsed, sparking fears that residents could be trapped.
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"It is possible that some people are trapped under the rubble," Dmitry Ulanov, spokesman for the regional branch of the emergency situations ministry, told AFP.
A source in the ministry told Interfax news agency that two people found under the debris did not show signs of life but Ulanov said he could not confirm the report.
The ministry said emergency workers were still battling to put out the fire sparked by the blast.
Residents will be temporarily relocated to local schools, the emergency situations ministry said.