Labouring through sub-freezing temperatures, Russian rescue workers were digging into a sprawling heap of jagged rubble from a collapsed apartment building when one heard the faintest sound.

It was the sound of life.

On Tuesday, to everyone's delight and surprise, they pulled a baby boy out of the rubble alive, nearly 36 hours after the disaster that blew apart his home.

The building collapse in the Russian city of Magnitogorsk before dawn Monday has so far killed at least nine people, and 32 people who lived in the building have still not been accounted for.

The collapse followed an explosion that was believed to have been caused by a gas leak.

The boy, an 11-month-old named Ivan Fokin, was in extremely serious condition, officials said, with fractures, a head injury and suffering from hypothermia after his ordeal in temperatures around –20 C.

A section of the apartment building collapsed after a gas explosion Monday in Russia's Urals city of Magnitogorsk. (STR/AFP/Getty Images)

He was being flown to Moscow late Tuesday in a desperate attempt to save his life.

Although Ivan's prospects for survival appeared dire, his father, Yevgeny, told RT satellite TV channel Tuesday's rescue was "a New Year's miracle."

The father was at work when his wife phoned to say the building had collapsed. She escaped the rubble with a three-year-old son, Russian news reports said.

"I was sleeping on the couch with my older son, hugging him and the young one was sleeping in his baby bed," mother Olga Fokina said on Russian TV. "I and the older one fell down and quickly got out and I didn't know what happened to the baby bed afterward."

Rescue worker Pyotr Gritsenko said on Russian television that baby's discovery came after one of the crew heard faint cries.

"They stopped all the equipment. He began to cry louder," but the crew couldn't find him, he said. A search dog was brought in and confirmed that someone was under the rubble, focusing the rescue effort.

"The child was saved because it was in a crib and wrapped warmly," Boris Dubrovsky, the regional governor, was quoted as saying by the Interfax news agency.

The rescue operation, aided by powerful heaters and lights, was continuing overnight into Wednesday in the city about 1,400 kilometres southeast of Moscow.

Image from TV footage shows Emergency Situations worker carrying the baby saved from rubble of a collapsed section of an apartment building in Magnitigorsk southeast of Moscow on Tuesday. (Russian Ministry for Emergency Situations via AP)

Russian President Vladimir Putin visited the accident site on Monday and went to a local hospital, where he spoke to a 13-year old-boy who had head injuries and frostbite after spending an hour under the rubble.

"You will get well soon. You are a fighter," Putin told the boy, one of five people hospitalized from the building collapse.

Russian officials say the odds of finding anyone else alive in the debris look increasingly slim, given the extreme weather.