Kenya Red Cross said the building had been vacated before the collapse, and that rescue personnel were checking the area to make sure no one was in its vicinity when it came down.
Witnesses and officials in Kenya say an eight-story building has collapsed in a low-income area of Nairobi.
Pius Masai, deputy head of communications at the National Disaster Management Unit, said: “It is believed that some people may have been trapped”.
“Rescue efforts are ongoing”, he said in a statement, and appealed for people with access to “cutters, drillers and any other extrication equipment” to help with the search.
Local media reports that some people re-entered the building apparently to collect their belongings when it caved in, possibly trapping them.
Police and emergency service providers leading the operation said it would take long to dig through the rubble to confirm the claims. Police said they do not know how many people are trapped.
A total of 49 people died a year ago when another building collapsed during heavy night-time rain in Huruma estate, Nairobi.
But at 6pm, Mr Nemati said, his wife called him saying tenants had been ordered to leave immediately as the building was coming down.
The Daily Nation reported that a woman and her children had refused to leave their home and are among those unaccounted for.
Kenya has witnessed similar tragedies in the past.