MOSCOW—Authorities say a gas explosion in an apartment building in the Siberian city of Novosibirsk has killed at least 12 people, including a 2-year-old child.

The Thursday morning explosion caused two entrances of the five-story building to collapse, with 30 apartments destroyed by the ensuing fire.

Russian news agencies cited the city’s emergency services as saying 12 people were killed, nine others were hospitalized and the fate of three people believed to be in the building was unknown.

Novosibirsk is the biggest city in Siberia and Russia’s third-largest by population, with about 1.6 million people.