The London Fire Brigade confirmed that six engines attended the scene. It was unclear how large the blast was or how many residents were injured.
A woman living on Drinkwater Road where the building collapsed said she heard a "big explosion" at about 8am.
Behnaaz Shah said: "I was on my way out when I heard the explosion and I came out to have a look. The building opposite looked like it had exploded and there was fire coming out of it."
She said the building's roof had caved in and a section of Drinkwater Road had been cordoned off.
London Fire Brigade said around 40 people had to be evacuated from the residential block.
A spokesman added: "Early reports suggest there has been serious structural damage to the building."