Rescuers are searching for survivors in the central Russian city of Magnitogorsk, where an explosion devastated a block of flats, killing at least four people.
Officials in the city, in the Urals region, say a gas leak caused the blast. About 40 people are missing.
President Vladimir Putin has arrived in the city to assess the situation.
The building was home to 120 people, and 48 flats collapsed in the blast.
"I woke up and felt myself falling," one witness told Russia television, according to news agency AFP. "The walls were gone. My mother was screaming and my son had been buried."
Another witness spoke of a "wave of fire" which followed the blast.
Rescue workers are now in a race against time to find survivors, as the daytime temperature in the industrial city is around -17C, falling further overnight.
Few, if any, are expected to be pulled from the rubble, but rescue workers are due to work through the night in the hope of finding people alive.
Russia's Vesti news channel says officials have accounted for more than 50 of the block's residents, including the victims.
At least 16 survivors have been evacuated from the scene, including seven children. Five injured people, including two children, were taken to hospital, according to officials.
President Putin, who came to see the rescue operation for himself, described it as a "tragedy", adding that those affected would get every assistance.
"First of all, I would like to express my condolences to the families of the victims, and to assure those affected that we will do everything to help and support them," he said.
The blast, at 06:02 local time (01:02 GMT) is thought to have ripped through the first floor, which houses some offices, and the seven storeys above then collapsed. Nearby flats were also damaged.
There are concerns another two sections of the block, which was built in 1973, could collapse.
A criminal investigation has been opened into the disaster.
The city is about 1,695km (1,053 miles) east of Moscow.