Nairobi  - Some 40 people are missing and one person is dead two days after a blaze ripped through a supermarket in the Kenyan capital Nairobi.

The fire broke out on Wednesday and quickly engulfed the downtown branch of Nakumatt, a popular supermarket chain.

Nakumatt on Wednesday evening issue a statement saying all of its staff and customers were safe, but the claim now seems to have been premature.

The Kenyan Red Cross said it had registered the names of 40 missing people - five of them members of the Nakumatt staff - and was counselling their families.

The blaze appeared to break out quickly, trapping many people on the upper floor of the supermarket. Some survivors reported hearing explosions before the flames raced through the building.

So far, only one person has been confirmed dead: a staff member who died of spinal injuries after jumping from the roof of the building.

However, relatives of the missing say they their loved ones had called to say they were trapped.

Janet Odera, 40, one of the missing, called her brother Simon as the flames closed in around her, the Daily Nation reported.

"She called me and my brother Charles to say she was trapped in the fire and choking," Simon Odera said. "She said they were dying and asked us to pray for her."

The building continued to smoulder all throughout Thursday, sending a spiral of noxious smoke up over the Nairobi rooftops.

Fire services were late Thursday still unable to enter the building, but were expected to begin the grim task of searching for bodies early on Friday. (dpa)