At least four people died after an apartment block in Russia's Ural Mountains collapsed following a gas explosion, the Russian Emergencies Ministry said Friday. Fifteen people have been rescued, but dozens more are feared trapped under the rubble.

Photos published by Russian agencies showed a large part of the five-story prefab building, in the industrial city of Nizhny Tagil, leveled to the ground.

According to Russia's Emergencies Ministry, the explosion of a natural gas tank was the likely cause. Residents of neighboring blocks told Russian news outlets that they heard a loud blast immediately before the collapse on Thursday.

A rescue operation involving heavy equipment and over 500 emergency workers continued to comb the debris on Friday, according to Russian emergency officials. They said they had so far freed 15 people, including seven children, as well as a cat. Most of those pulled from the rubble were hospitalized, including a girl who is in serious condition, local health officials said.

Russia's law enforcement launched a criminal probe into the incident, suspecting a health and safety failure.