The incident occurred near a Somali-dominated area of the city. No immediate claim of responsibility was recorded.
Police, however, said attack mirrored some explosions in 2012, which were blamed on Somali militants.
“So far, I can confirm that four people have died in the explosion,” the Nairobi County Police Commander, Benson Kibui, told Reuters, without giving details.
Another police officer described it as a grenade attack.
At the scene in Pangani, an area next to Eastleigh, which is home to many Somalis, a Reuters reporter saw a destroyed minibus and metal, glass and other debris on the street. A car nearby was also damaged.
Kenya Red Cross and other ambulances were there. St John Ambulance reported on its Twitter account that 15 people were taken to one hospital and three died from their injuries.
On Friday, there was a grenade atttack on a market in Wajir, in northeast Kenya, near the Somali border. In that attack, police said masked men hurled two grenades, killing one person and injuring others.
Last year a series of such attacks in and around the Eastleigh area of Nairobi were blamed on Somalia’s al Shabaab Islamist group. The group staged an assault on a Nairobi shopping mall in September, which killed 67 people.
Al Shabaab said its gunmen carried out the mall attack to force Kenya to withdraw troops from Somalia, where they are part of an African peace-keeping mission battling with the Islamists.
Philips added that as a part of ways to solve the Boko Haram menace, there was the need for government to ensure equal access to opportunities across ethnic and religious lines.
He said, “The Boko Haram issue has to become a Nigerian issue. It doesn’t matter what part of the country I live or my religion. I do not believe that any true Muslim believes that this is what should be happening. I believe that we should stand up to be counted.