"There are dead and injured," the regional prosecutor's office said, adding it was working to clarify the number of casualties.
State television broadcast footage of the collapsed corner of the high-rise building in Izhevsk, some 940 kilometres east of Moscow.
A representative of the Investigative Committee, Vera Filippova, said in televised remarks that preliminary information pointed to a gas explosion as the reason for the collapse.
Flats on all nine floors have apparently been reduced to debris, with dust rising from the pile, footage showed.
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Thirty-two flats are believed to have been damaged.
"There was the sound of such a loud blast that even the building shook," a witness named Olga who lives in a neighbouring building told state television.
The damaged building is a typical Soviet-era residential block made of concrete panels.