Rescuers have pulled an infant boy alive from the rubble of an apartment building, some 35 hours after a collapse that has killed at least seven people and left dozens missing.

They found the baby on Tuesday after hearing cries amid the debris.

A section of the 10-storey building in Magnitogorsk, Russia collapsed on Monday following an explosion believed to have been triggered by a natural gas leak.

The baby, who officials initially said was a girl, was seriously injured and his recovery prospects were unclear.

The boy's mother survived the collapse and went to a local hospital to identify him, the state news agency, Tass, reported, citing the regional emergencies ministry. The child will be evacuated to Moscow by aeroplane for further treatment, Tass said.

Thirty-seven residents of the building are still missing, the regional emergency minister said on Tuesday.

Another 10 people had to be dug out of the rubble of the building, Tass reported, while 16 people were evacuated from the building without injuries.

According to Tass, the vice governor of the Chelyabinsk region, Oleg Klimov, said there were 79 residents of the building.

At the emergency site, some 470 people with 70 excavators and vehicles searched through the rubble.

Hopes of finding survivors were dimmed by the harsh cold; temperatures overnight were around minus 18 Celsius.

"The child was saved because it was in a crib and wrapped warmly," said regional governor Boris Dubrovsky according to the Interfax news agency.

Rescue crews had temporarily halted their search while workers tried to remove or stabilise sections of the building in danger of collapse.

Five people were hospitalised with injuries from the collapse in the city located about 1,400km southeast of Moscow, the emergency ministry said.

In a separate holiday period disaster, seven people including a couple and their three children died in a house fire in the town of Orsk, about 1,500km southeast of Moscow, Interfax said. The fire early Tuesday is believed to have been caused by an electrical short-circuit, the report said.

Explosions in Russian apartment buildings occur from time to time. Magnitogorsk is considered one of the worst-polluted cities in the world due to its metallurgy and steel industry.