"I was on the sixth floor. I woke up because of a loud roar," said Svetlana Andrievskaya, speaking from a hospital bed. "I could see the room but there was no wall."
Most residents were sleeping when the blast ripped through the building in the far northern White Sea port of Arkhangelsk at about 3 am (midnight GMT) on Tuesday, the Emergencies Ministry said.
"It is difficult to say precisely, but about 20 people may be still under the rubble," an Emergencies Ministry spokeswoman said, adding "Searching through the rubble is a long process, it could go on for two days."
Rescuers pulled 23 people alive from the section wrecked in the blast and officials vowed to keep searching for survivors.
Itar-Tass news agency said more than 600 rescuers were picking through mounds of steel and concrete. The city's entire fleet of cranes was drafted in to help shift piles of rubble.
Sofas and smashed furniture littered the search area and clouds of dust hung over the area where 36 shattered flats had once stood.
"That section caved in just like a house of cards, leaving hardly any air pockets where people could be"
spokesman, Emergencies Ministry
Extra rescuers, aided by sniffer dogs, flew to the site from Moscow and other cities at first light. Crews stopped for a minute's silence every half hour to listen for signs of life.
Officials said the blast was almost certainly caused by a gas leak, common in the decrepit, crumbling apartment blocks that house millions of Russians.
News agencies said police had reported other gas leaks overnight after vandals stole gas distribution equipment. They said police were seeking two tramps seen carrying tools near the area before the blast.
Officials said the use of prefabricated concrete blocks in the construction of the building left little chance of anyone surviving such a collapse.
"That section caved in just like a house of cards, leaving hardly any air pockets where people could be," Tass quoted Emergency Ministry spokesman Viktor Beltsov as saying.
A gas explosion last month in a Siberian cafe killed 18 and on Sunday the historic Manege building, next to the Moscow Kremlin, burnt down after a suspected electrical fault.