Death toll rises to 12 following the London apartment building fire.

Twelve people have been confirmed dead following a fire which ripped through a 24-storey housing block in west London in the early hours of Wednesday morning.

Flames raced through the high-rise Grenfell Tower block of flats in the north Kensington area after taking hold before 1am. Witnesses reported many residents desperately calling for help from windows of upper floors.

Police said they are expecting the death toll to rise further, as there were up to 600 people thought to have been inside the tower of 130 flats at the time of the blaze.

Eighteen people are reportedly receiving critical care after 74 people were taken to hospital.

More than 200 firefighters, backed up by 40 fire engines, fought for hours to try to bring the blaze, one of the biggest seen in central London in recent years, under control.

The cause of the fire was not immediately known.

The block had recently undergone an STG8.7 million ($A14.6 million) refurbishment of the exterior, which included new external cladding, replacement windows and curtain wall facades.

Plumes of black smoke billowed high into the air over London for hours after the blaze broke out. Residents rushed to escape through smoke-filled corridors after being woken up by the smell of burning.

London Fire Brigade said the fire engulfed all floors from the second to the top of the 24-storey block. There were reports that some residents threw themselves out of windows to escape the flames.

“In my 29 years of being a fire fighter, I have never ever seen anything of this scale,” London Fire Brigade Commissioner Dany Cotton told reporters.

More than 10 hours after the fire broke out London fire brigade said it was still working to bring the fire under control, though the building was not in danger of collapse.

A witness told Reuters she feared not all the residents had escaped the fire. Some were evacuated in their pyjamas.

“I looked through the spy hole and I could see smoke everywhere and the neighbours are all there. There’s a fireman shouting ‘get down the stairs’,” one of the block’s residents, Michael Paramasivan, told BBC radio. “It was an inferno.”

“As we went past the fourth floor it was completely thick black smoke. As we’ve gone outside I’m looking up at the block and it was just going up. It was like pyrotechnics. It was just unbelievable how quick it was burning.”

Ash Sha, 30, who witnessed the fire and has an aunt in the building who managed to escape from the second floor, said the local council had recently renovated the tower.

“They cladded the outside and insulated the inside,” Sha said. “The insulated material is very similar to sponge so it crumbles in your hand. This was just done to tart it up and match the nearby building.”

London mayor Sadiq Khan said the fire raised questions over safety of high-rise blocks like Grenfell Tower.

“Across London we have many, many tower blocks and what we can’t have is a situation where people’s safety is put at risk because of bad advice being given or if it is the case, as has been alleged, of tower blocks not being properly serviced or maintained,” he told BBC radio.

The local council of Kensington and Chelsea, which owns the block, said the causes of the fire would be fully investigated.

Police closed the A40, a major road leading out of west London, while some parts of London’s underground train network were closed as a precaution.

The Australian High Commission in London is making inquiries to determine whether any Australians have been affected by the fire.