One unidentified person and a one-year-old child died in a fire in an apartment in Johannesburg, Gauteng police said on Sunday.

Inspector Lorraine van Emmerik said a fire broke out at the corner of Commissioner and Main streets on Saturday evening.

The mother of the infant had left her apartment and the child on Saturday evening.

The fire broke out in the apartment but neighbours could not save the child because the apartment was locked.

"Neighbours at the time tried to gain entry but due to the condition of the building, it collapsed," Van Emmerik said.

Fire-fighters extinguished the fire and searched the rubble and found the body of the child at around 10.00pm on Saturday.

Residents told rescue workers that another person, gender and age unknown, had been inside the building when it had collapsed.

Fire-fighters found the body of the adult in the rubble at around 1.00am on Sunday.

The cause of the fire was unknown but an investigation into the incident was being conducted.

Police called on anyone with information on the incident to contact the Johannesburg Central police station on 011-497-7000 and to ask for the Detective Branch, said Van Emmerik.