Four people have also been taken to hospital.
Sixteen people including seven children have been evacuated.
National television broadcast footage of the mangled heaps of concrete as hundreds of rescue workers combed the debris in temperatures of minus 18 Celsius.
Temperatures in Magnitogorsk were expected to plunge to minus 23 Celsius on New Year's night, the biggest holiday of the year in Russian Federation.
Russian President Vladimir Putin traveled to Magnitogorsk after being notified of the tragedy.
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Sixty-eight residents were still missing from the high-rise, home to some 120 people, Governor of the Chelyabinsk Region Boris Dubrovsky said in a post on Telegram, according to Tass.
Magnitogorsk is a city of about 400,000 just over 1,000 miles from Moscow.
The explosion shattered windows of nearby buildings, according to a local resident speaking on Echo of Moscow radio.
The residents have been evacuated.
The head of the Tajik diaspora in Magnitogorsk, Abulmajid Sharipov, told RFERL that a Tajik family of five was living in an apartment in the collapsed part of the building. Investigators opened a criminal probe into the accident, with the FSB security service confirming the blast had been the result of a gas explosion.
There have been several similar incidents in Russian Federation in recent years due to aging infrastructure and poor safety regulations surrounding gas usage.