lScores of people are feared trapped under rubble following the collapse of a six-storey building at Tassia estate in Embakasi, Nairobi, on Friday morning.
The block came tumbling down minutes after 8am, as some tenants prepared to leave their homes for work.
Those who escaped unhurt said the ground floor sank and then the others followed in due sequence.
They said there had been no signs of the building's weakness so many were caught unawares.
However, the house is one of many crammed developments in a swampy area with a high population, much like the rest of Embakasi.
So far, at least 17 people have been rescued, some with serious injuries, and taken to nearby hospitals by emergency responders.
The responders are from Kenya Red Cross Society, St John Ambulance, Nairobi County's fire department, Kenya Police and the Kenya Defence Forces’ Disaster Response Unit (RDU).
Red Cross said in an update at about 4pm that 11 injured people were taken to Mama Lucy and Mater hospitals and that 12 people had reported missing. The society said 22 families lived in the building.
Public Works Principal Secretary Gordon Kihalangwa, one of the government officials who visited the scene, said the block had 46 houses.
It was not immediately clear how many tenants were inside when disaster struck but rescuers feared some were trapped under the rubble. Police and KDF men used sniffer dogs to locate the victims.
The owner of the building, identified only as Mama Kanyoni, was expected to give tenancy records but she was nowhere to be found.
The collapsed building destroyed an adjacent single storey iron shanty with several houses.
The cause of the building's collapse was not immediately clear but heavy rains have been pounding Nairobi and other parts of the country.
The meteorological department said the rains will continue until the end of December, with intermittent episodes of heavy and light rainfall up to the fourth week of the month.
The department urged the public to be on the lookout for landslides and overflowing river banks.
Mr Kihalangwa said search and rescue operations were hampered by the multitudes of people who gathered at the scene.
Responders also experienced difficulties reaching the scene as access roads in the estate are in poor conditions.
The presence of underground and surface water, due to heavy rains, also hampered their efforts
The floods dashed hopes of finding survivors, with rescuers reporting that a trapped woman they were communicating with was no longer responding.
“But we can hear a baby crying,” a Red Cross volunteer said.
Many buildings have collapsed in Nairobi and other parts of the country in the past, raising concerns about the quality of construction.
In Nairobi, such incidents have taken place in Zimmerman, Roysambu, South B, Huruma and Kaloleni.
In 2016, President Uhuru Kenyatta ordered an audit that covered most parts of Eastlands, Dagoretti, Kasarani, Zimmerman, Roysambu, Githurai 44 and 45, Garden Estate, Thome, and Kilimani.
The report concluded that buildings collapse because of poor quality of concrete, poor foundation and use of substandard building stones.