'Huge bang'
Television pictures showed emergency rescue workers and machinery searching for survivors in the smouldering rubble of the apartment building as shocked locals looked on.
A witness from a nearby apartment told Russia's Channel One television: "A huge bang woke me up and glass began to shower down on me. I went onto the balcony and I heard screaming. Everything was on fire."
A woman who lived in the building told the Russian Vesti-24 television station: "We found our grandmother but we are still missing our grandfather and our seven-year-old girl."
The explosion happened shortly after midnight on Wednesday.
Such incidents have become commonplace in Russia's ageing apartment buildings.