London Fire Brigade has just confirmed "a number of fatalities" but it is still unknown how many people have perished in the blaze.

Thirty people have been taken to hospital and 200 firefighters remain at the scene, attempting to extinguish the blaze.

There is 120 flats in the building but authorities do not know how many people were inside at the time.

Many residents say they did not hear fire alarms when the blaze began, which spread rapidly, consuming the entire building within minutes.

Fire has engulfed a 27-storey building called Grenfell Tower, in a large housing estate in London.

More than 200 firefighters are battling the inferno.

The fire broke out at 12.54am (London time).

According to several witnesses, the blaze started on one side of the tower block, before sweeping around the building and engulfing it in flames from the second to the top floor.

The building has been gutted.

People were trapped in the building, while others jumped for their lives, according to reports.

The Mayor of London, Sadiq Khan, has declared the fire a "major incident".

The 27-storey building is located near White City and Shepherd's Bush, west of London's CBD.

The building is still standing, but huge sheets of cladding are falling off the tower.

The falling debris is reportedly causing nearby flats to be evacuated.

10pm Wed (AEST): Police have confirmed "at least" six deaths.

More than 60 people have been taken to hospital. Twenty of those are in critical care.

Police say a "number of people are being treated for a range of injuries" including smoke inhalation.

Witnesses have spoken of hearing screams and seeing some people escaping using bedsheets as improvised rope ladders.

CNN has reported that one witness saw people jumping from the burning building.

London Fire Brigade says there is "no known cause" at the moment for the blaze.

There have been previous warnings about the risk of a "major fire disaster" in the building, following an earlier incident in 2013.

The building recently underwent a $14.5 million refurbishment, with insulated exterior cladding installed.

It is too early to say whether any construction materials contributed to the fire, however combustible cladding has contributed to the spread of other major high-rise fires around the world.

In July 2009, six people were killed and more than 20 injured in a blaze at Lakanal House, a tower block in Camberwell, south-east London.

In May 1968, four people were killed and 17 injured when Ronan Point, a tower block in Newham east London, partially collapsed after a gas explosion, which blew out several load-bearing walls.

- with Telegraph, London