LONDON, United Kingdom (AFP) — At least six people were killed Wednesday when a massive fire tore through a London apartment block in the middle of the night, with witnesses reporting terrified people had leapt from the 24-storey tower.

Parents wrapped wet towels around their children as they desperately tried to escape from the Grenfell Tower, with at least one person seen waving a piece of white cloth from an upper floor as flames ravaged the building.

“I can confirm six fatalities at this time but this figure is likely to rise during what will be a complex recovery operation over a number of days,” said police commander Stuart Cundy, describing the fire as “truly shocking”.

The ambulance service said 50 people had been hospitalised.

The alarm was raised just before 1:00 am and within an hour flames had engulfed the entire block. Residents claimed the fire was spreading on the exterior of the tower, which had been covered in cladding in a major refurbishment completed last year.

More than 10 hours on, flames could still be seen inside the charred building as 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.

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

Nick Paget-Brown, leader of the Kensington and Chelsea local authority, said “several hundred” people would have been inside the block.

London Fire Brigade chief Dany Cotton said the building’s structure 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 firefighters had managed to reach 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.

Witnesses said they heard screaming from the upper floors as the flames raced up the tower. “I saw people jumping out of their windows, the building was literally on fire, the ambulances, the police. It was horrendous,” Khadejah Miller, who was evacuated from her home nearby, told AFP.

Adi Estu, 32, who was in her pyjamas and a coat, said: “I saw people flashing their lights for help, families flashing their mobile phones like a torch. But the smoke covered them and then the fire destroyed everything.

“We saw them dying. How can you forget that?”

Hanan Wahabi, 39, said she escaped with her husband, 16-year-old son, and eight-year-old daughter, 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 a community centre that is serving as a hub for displaced residents.

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

“The fire was on the outside, not the inside,” he said.

Frantic families 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 more than 200 firefighters had been working on the blaze.

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.

“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,” read a blog post by the Grenfell Action Group.

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,” he told the BBC.

Angus Law, from the Centre for Fire Safety Engineering at Edinburgh University, said the blaze seemingly had similarities with other recent fires worldwide.

“It appears that the external cladding has significantly contributed to the spread of fire,” he said.

He said regulations for tall buildings were intended to prevent the spread of fire, so if it does occur, “the consequences are often catastrophic”.

The fire comes 11 days after eight people were killed in a terror attack on London Bridge and the nearby Borough Market.