The Somali Islamist militant group, al Shabaab, has already claimed responsibility for the assault.
At least one explosion and sustained gunfire had sent workers fleeing for their lives at an upscale hotel and office complex in Nairobi.
Police chief, Joseph Boinnet, says the assault is a suspected militant attack and the gunmen may still be at large.
Kenya has often been targeted by al Shabaab, who killed dozens of people in a shopping centre in 2013 and nearly 150 students at a university in 2015.
