A massive fire tore through a 27-storey apartment block in London before dawn on Wednesday, causing fatalities and injuring dozens, with terrified residents trapped inside.

London Fire Brigade chief Dany Cotton said there had been "a number of fatalities" in what she called an "unprecedented" blaze.

"I cannot confirm the number at this time due to the size and complexity of this building," she told reporters at the scene in west London.

"This is an unprecedented incident. In my 29 years of being a firefighter, I have never ever seen anything of this scale," she said.

The ambulance service said 50 people had been hospitalised.

Witnesses said they had seen some people fall or jump from the stricken Grenfell Tower and at least one resident waving a piece of white cloth from one of the upper floors as flames ravaged the building.

The alarm was raised just before 1:00 am (0000 GMT) and within an hour flames had engulfed the entire block, which contains 120 flats.

Residents said the fire was on the exterior of the tower, which had been covered in cladding in a major refurbishment completed last year.

Nearly 10 hours on, flames could still be seen inside the charred building as a thick, black smoke filled the sky.

London Mayor Sadiq Khan said firefighters were only able to reach the 12th floor at the height of the blaze during the night.

"A lot of people are unaccounted for," he told Sky News television.

Cotton said the building's structural integrity was being monitored and it was stable enough for fire crews to work inside.

"We rescued a large number of people from inside the building very early on," she said, adding that she had spoken to firefighters who had reached the 19th and 20th floors.

"We are making steady progress."

Large pieces of debris could be seen falling from the building, a 1970s local authority-built block in the working-class north Kensington area.

- 'I just ran' -

Witnesses said they heard screaming from the upper floors as the flames raced up the tower in the night.

"They were trapped. They couldn't come downstairs, especially from the top floor... people have been burned," a witness identified as Daniel told BBC radio. 

"I have seen it with my own eyes. And I have seen people jump."

Hanan Wahabi, 39, told AFP she escaped with her husband and son, 16, and daughter, eight, but feared for her brother and his family who live on the 21st floor.

"Last time I saw him they were waving out of the window, his wife and children," she said, sitting outside the Rugby Portobello community centre, acting as the hub for displaced residents.

"I've not heard from them since, the phone is not going through," she said.

Wahabi's husband Salah Chebiouni, 45, said he was in the kitchen when "I saw fire outside my window".

"Fire on the ninth floor -- that's wrong. The fire was on the outside, not the inside. From the windows, not the door," he said.

Eddie, 55, who lives on the 16th floor, said he ran out of his house with a wet towel wrapped around his head as smoke engulfed the flat.

"You couldn't see anything. I just ran down the stairs... Loads of people haven't got out of the building," he said.

Another witness named as Jody Martin told the BBC that he battled his way his way to the second floor only to encounter choking smoke.

"I watched one person falling out, I watched another woman holding her baby out the window," he said.

Frantic families at the scene attempted to call their loved ones, fearing they could be stuck inside, and were being directed by police to a nearby restaurant where some of the injured were being treated.

The fire brigade said 40 fire engines and 200 firefighters had been working on the blaze.

"Fire is from second to top floor of 27 storey building," the fire service said on Twitter.

Police cleared nearby buildings due to fears about falling debris.

- Fire risk warning -

The apartment block was built in 1974, but had recently undergone a major refurbishment, including a new heating and hot water system and new cladding on the outside.

The refurbishment cost £8.7 million ($11 million, 9.9 million euros) and was completed in May 2016.

Local residents had warned a year ago about a potential fire risk caused by rubbish being allowed to accumulate during the improvement works.

"This matter is of particular concern as there is only one entry and exit to Grenfell Tower during the improvement works," read a blog post by the Grenfell Action Group.

"The potential for a fire to break out in the communal area on the walkway does not bear thinking about as residents would be trapped in the building with no way out," it said.

Some residents said the official advice was that people should stay inside in the event of a fire.

Mayor Khan said the advice would be looked into.

"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 it's been alleged, of tower blocks not being properly serviced and maintained," he told the BBC

Source: AFP