A teenage boy who jumped from the window of a burning building has died from his injuries.
The 13-year-old was critically injured as he tried to escape the fire in St John's Wood, north-west London, in the early hours.
Firefighters had earlier discovered the body of a girl, believed to be aged five or six, inside a third-floor flat in Cunningham Place.
Police said the two dead children were siblings.
Three people, the dead children's brothers aged 12 and 15, and their father, aged 44, were rescued by firefighters.
They were treated in hospital for minor injuries including burns and smoke inhalation.
A Metropolitan Police spokesman said a 32-year-old man was arrested near the scene and was being questioned over the circumstances of the blaze.
Investigators were today examining the badly damaged third-floor property to try to establish the cause of the fire.
Emergency services were called to the four-storey block of flats shortly before 3am. Up to 20 firefighters spent more than an hour battling to bring the blaze under control.
A Metropolitan Police spokesman said: "Inquiries into the cause of the fire by police and London Fire Brigade are ongoing at this early stage.
"One man has been arrested in connection with the incident."
A London Fire Brigade spokesman said: "Firefighters discovered the body of a young female child at the flat.
"Two adult males and two male children were rescued by firefighters wearing breathing apparatus. A male child jumped from a third-floor window.
"The fire badly damaged the third-floor flat. There are four floors in the building. The brigade was called at 2.54am and the fire was under control by 4.20am."
Daniel Astaire, of Westminster City Council, said: "This was a terrible tragedy and our thoughts are with the family at this difficult time.
"We will assist the police and fire service in any way we can with their inquiries."