The nine were evacuated to Neema Hospital off Thika Road after the incident that occurred at about 11am.
Kasarani OCPD Francis Sang’ said at least six workers were still trapped under the rubble.
Speaking at the scene, Sang said the collapse may have been due to faults related to a hurried construction.
A vehicle was also damaged when the six-storey residential building collapsed.
Kasarani AP and General Service Unit officers and the National Youth Service are part of the rescue team.
In January, Nairobi Governor Evans Kidero issued a warning to contractors who did not meet the approved construction standards.
Kidero made the remarks following the collapse of a building in Huruma in which five died, and another in Makongeni in which four were killed last December.
Raw Video: Nine workers injured as Roysambu Building Collapse