Moscow, Jan 2 (IANS): The death toll from the collapse of an apartment building wrecked by a New Year's Eve gas explosion in Russian city of Magnitogorsk rose to 21 on Wednesday, officials said.

Over 20 people are still unaccounted for, a spokesman of the Russian Emergencies Ministry told TASS news agency.

The blast ripped through the first floor of the building on December 31, bringing a section of the 10-storey apartment block crashing down. In all, 48 flats -- home to 120 people -- were wrecked.

A 11-month-old baby boy was found alive in the rubble on Tuesday and reunited with his mother. His condition was serious but not life-threatening. He was in a Moscow hospital and will not undergo surgery for now, the BBC reported.

The rescued baby, called Ivan, spent 35 hours in the freezing cold and was reported to have severe frostbite to his limbs, a head injury and multiple leg fractures.

A day of mourning was marked in Magnitogorsk, in the Urals region about 1,695 km east of Moscow. Rescuers braved temperatures that fell as low as minus 27 degrees Celsius.

Investigators did not found any traces of explosive at the scene and the State Security Service was treating it as a gas leak accident.

However, a criminal investigation was opened for suspected criminal negligence.