London: Neighbours have described the horror of watching children trapped on the top floors of a 24-storey apartment block screaming for help and for their mothers as flames engulfed the building.

Police say the death toll stands at 12 but is expected to rise after a fire broke out and quickly spread through the Grenfell Tower in west London just before 1am on Wednesday. London ambulance said 74 people had been treated across six hospitals in London. 20 are are in a critical condition.

Firefighters are scouring the charred remains of the housing estate after a blaze one witness described as a like a "match to paper".

Ten hours after the fire broke out, flames were still visible halfway up the building and an enormous smoke cloud could be seen right across London, dominating the otherwise clear blue sky.

Metropolitan Police commander Stuart Cundy said the fire was "truly shocking".

"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," he said. "Many others are receiving medical care.

"It is likely to take some time before we are in a position to confirm the identity of the victims."

He said it was too early to speculate on what had caused the fire.

Despite fears the blackened, still-burning shell of the tower could collapse, London fire commissioner Danny Cotton said the building was safe for firefighters to search and crews had reached the 20th floor during the blaze.

"We rescued large numbers of people," she said.

A nearby resident called Mark first saw the blaze from the window of his bedroom on St Anne's Road. He said the sound of helicopters, sirens and screams woke him and he initially thought it was a terrorist attack.

"I heard screaming - kids - I could hear them screaming, I could hear the words specifically 'mum' and I could hear women screaming and I went 'No, that's not a terrorist attack'," Mark, 45, said.

He raced to Latimer Road to help but said by the time he got there the building was a "tower inferno".

"It was burning like paper, it was getting worse by the second.

"I saw people up on the second-from-top floor. They had the lights on their phones on and they were shining their lights on their phones through the windows so people could see them," he said.

"I focussed on a family on the top left-hand corner. I know there was a kid there, I could tell by the shape."

Asked if there was any chance they could have been rescued or escaped, he said "not a chance in hell".

"The casualty figures are going to be bad," he warned.

Another witness, Mohammed, said he saw two people jump from the 10th to 15th floor. He says he has still not heard from his friends - two families, one of three and one of five, who live on the 18th floor.

"No word, no answer," he said.

He said he had scoured the three community centres established for displaced residents and worried family and friends and feared the worst.

He said a relative of the family of five had been phoned and told their three children had not turned up to primary school.

Harrowing accounts are emerging of residents' desperate attempts to escape.

Samira Lamrani said she saw a woman trying to save a baby by dropping it from a window of the ninth or 10th floor.

She told the Press Association: "People were starting to appear at the windows, frantically banging and screaming.

"The windows were slightly ajar - a woman was gesturing that she was about to throw her baby and if somebody could catch her baby.

"Somebody did, a gentleman ran forward and managed to grab the baby."

Another resident, called Zara, told LBC radio: "One woman actually threw her son out of the window. I think he's OK.

"I think he might have just had some broken bones and bruises."

Attention turns to controversial cladding

Locals are speculating that the building's recent renovation - the addition of exterior cladding - caused the fire's intensity.

"It was burning like paper. It wasn't the building, it wasn't the structure; that cladding - it was just like throwing fuel on the fire," Mark said.

Mohamed said the fire took about 10 minutes to travel from the eighth floor to the top of the building.

"Because of these new panels, they're the things that ignited, I could actually see the panels on fire," he said.

Sam, 23, was just returning to his home on a street near Latimer Road when the fire broke out.

"The whole thing just went up, it took off like a match to paper," he told Fairfax Media.

"Whole panels were coming off in flame, [firefighter] hoses could reach only halfway up the building, and whole bits of the facade from the top, these panels were literally falling down, it was horrible.

"There are so many people who live there who've been displaced, it's shocking."

London mayor Sadiq Khan vowed to "get all the answers" to the "great many questions" as to the cause of the blaze.

Overwhelmed by generosity

At least a dozen women were spotted heading towards local charity The Clement James Centre carrying bags of clothing and other goods for residents left homeless by the fire.

By mid-morning the centre said it had been overwhelmed with the generosity of donations and could not accept any more.

Follow Latika Bourke on Facebook