Three people have been confirmed dead after a five-storey building collapsed in Huruma early Sunday morning.
According to Mr Pius Masai, the deputy director National Disaster Management Unit (NDMU), one person was found dead in the debris while another died on the way to the hospital. Three others have been hospitalised.
Mr Masai said many of the tenants of the ill-fated building left when it developed cracks shortly before midnight on Saturday.
But he said the number of building's occupants has not been established.
Tenants in the nearby buildings have been evacuated after rescuers declared the area a dangerous zone.
"We have enough equipment and personnel. What is delaying is entrance to the scene," said Mr Masai.
The collapsed building was adjacent to a primary school.
Rescue teams had to demolish several structures to access the site because it is in a densely populated area.
Already, the National Construction Authority has started inspecting nearby buildings and ejecting tenants from structures with cracks.
Mathare sub-County Deputy Commissioner Patrick Mwangi said the collapsed apartment had been marked for demolition but the tenants sneaked back in and had been staying there for months.
So far, one of the 12 families that were living there illegally has been accounted for, according to Mr Masai.