The blast rocked the apartment complex on Monday at around 4am local time, completely collapsing one of its sections, videos from the scene show.
The disaster left at least three people dead, according to local officials. Another three, including a child, were hospitalized with injuries. At least 13 people have been rescued from the rubble.
Some 469 rescuers and 66 pieces of equipment have so far been deployed to the scene of the tragedy. Additional resources, including psychiatric specialists and K-9 units, are being brought from all over Chelyabinsk Region and neighboring Bashkortostan Region.
While the city has deployed its first-response teams, the Emergencies Ministry remains on a standby to fly four planes from Moscow to deliver more staff to the scene.
Some 110 residents were registered as living in the apartment complex, and the fate of 79 remains unclear, Deputy Governor Oleg Klimov told reporters.
The tragedy was caused by a gas explosion, the regional office of the Federal Security Service (FSB) confirmed.
President Vladimir Putin has been briefed on the situation, and has ordered the Emergencies Ministry chief to fly to the scene, while urging the head of the Healthcare Ministry to allocate all necessary resources.