Russia's emergencies ministry told the news agency RIA that four people had been recovered alive after Monday's blast from the nine-storey building in Magnitogorsk, an industrial city some 1700km east of Moscow in the southern Urals.
News agency TASS said 79 people were unaccounted for, quoting Chelyabinsk regional Deputy Governor Oleg Klimov.
The emergencies ministry said the blast, probably caused by a gas leak, damaged 48 apartments.
RIA said it ripped through the building about 6am (local time) when many residents were asleep. Monday is a public holiday in Russia.
There have been several similar incidents in Russia in recent years due to ageing infrastructure and poor safety regulations surrounding gas usage.
In 2015, at least five people were killed when a gas explosion damaged an apartment building in the southern Russian city of Volgograd.
