A gas explosion destroyed part of a five-story residential building in the Russian city of Naberezhnye Chelny 930 km (580 miles) east of Moscow, the emergency service said on Monday.
At least four people were wounded, two remained buried under the debris and more than 30 others were evacuated from the site, the emergency service said in a preliminary report.
Video footage showed the roof and part of two upper floors collapsed, while the lights were still on in other flats in the building.
Gas explosions are relatively common in Russia because of aging infrastructure and poor safety regulations.
In September, a gas blast at a residential building in the Russian city of Noginsk killed two people, while days later, three people were killed in another blast in a Russian village in the Lipetsk region.