“It was most probably an electric fault,” Iteere said, playing down fears it had been a bomb attack by Somalia’s Al-Qaeda linked Shebab insurgents.
Kenya has been hit by a wave of grenade attacks the police have repeatedly blamed on Somalia’s Al-Qaeda linked Shebab insurgents or its supporters.
Last month the hardline Shebab warned Kenya of revenge attacks for sending tanks and troops into Somalia in October.
Minutes after the blast the pro-Shebab Twitter site Al-Kataib reported a “huge explosion in Nairobi” although did not specifically claim any responsibility.
The blast was powerful enough to rip the tin roof off the building and smash windows across the street.
Wounded were carried from the explosion in a commercial building housing several small shops. Many appeared to have cuts from broken glass or flying debris.
A large plume of black smoke rose high into the air as ambulances rushed the wounded to hospital, but fire fighting trucks with sirens blaring extinguished the blaze soon after.
An AFP reporter at the site saw at least ten badly wounded people, and around a dozen more with lesser injuries.
The Kenyan Red Cross said a disaster response team had been sent to the blast on the central Moi Avenue in the commercial heart of Nairobi. An emergency medical centre was set up close to the blast site.
The pavement nearby was spattered with blood as people were loaded onto an ambulance.
Kenya has suffered a series of attacks in the past several months. Two separate grenade attacks on Saturday wounded at least eight people in northeastern Kenya, the restive region bordering war-torn Somalia.
Earlier this month attackers launched a deadly grenade attack on a restaurant in the port city of Mombasa.