Dense smoke billows from a badly damaged building after an explosion at a shopping centre in downtown Nairobi.

Sirens blare as emergency personnel reach the bloody scene.

At least 15 people were reported wounded, 11 of them critically.

Nairobi's Police Commissioner said blackened wires inside the centre indicate a possible electrical fault, but did not rule out a bombing.

One witness said the blast knocked him down.

(SOUNDBITE) (Kiswahili) EYEWITNESS WHO DECLINED TO GIVE NAME, SAYING:

"I was passing by when I heard a loud blast and it knocked me down, I saw a vehicle that was nearby thrown to one side. I just took off running"

There have been a string of deadly attacks in Nairobi and the port city of Mombassa since Kenya sent troops into Somalia in October to fight al Qaeda-linked militants.

Nairobi has blamed the al Shabaab militants, who merged with al Qaeda earlier this year, for the surge in violence.

Sarah Sheffer, Reuters