Footage on social media showed the flames on most floors, with witnesses reporting seeing people trapped in their units, while others were forced to jump from the apartment block, some from as high as 15 floors. About 600 people are believed to have been inside the tower's 120 flats when the blaze ripped through the building.
"This is going to be a long and complex recovery operation and I do anticipate that the number of fatalities will sadly increase beyond those 12".
By mid-morning in London, fire crews trying to clear the building had reached the 19th floor of the tower, searching room-by-room for victims or survivors.
London Fire Brigade said the fire engulfed all floors from the second to the top of the block which contained 130 apartments.
London Fire Brigade Commissioner Dany Cotton told CNN that in his 29 years of experience as a firefighter, it was his first time to see "anything of this scale". "Ten of our blocks are eight stories or above and while the fire risk assessment for each block is usually reviewed annually, we will be revisiting any conducted more than three months ago, as an extra precautionary measure to reassure residents".
A baby was caught by a member of the public after being dropped from a window as fire engulfed London's Grenfell Tower, a witness has said. We will be carrying out a review of these along with reviewing our fire compartmentation reports, which relate to restricting the spread of fires within buildings.
"I got dressed, went downstairs and saw a neighbor".
"This is an unprecedented incident".
Eyewitnesses described people trapped in the burning Grenfell Tower, in north Kensington, screaming for help and yelling for their children to be saved.
"They cladded the outside and insulated the inside", Sha said.
Since then, more than 64 people have been taken to hospitals, and 20 are in critical care units, the London Ambulance Service says.
"The buildings that I am seeing are very significantly non-compliant", he said. Others said they could see people inside using flashlights and mobile phones to try to signal for help from higher floors.
However, he spoke to her again at 2.15am and she said "smoke was coming into the flat".
"My thoughts are with the residents, their families, and people waiting for news of loved ones", he said.
London Fire Brigade said the cause of the fire is still being investigated, but several residents reported one man had said it started in his faulty fridge.
Tamara, one witness, told the BBC: "There's people, like, throwing their kids out: 'Just save my children, just save my children!'".
Mickey Paramasivan, who took his children out of the building after he woke up and smelled smoke, told Ritson they made it out with just a few minutes to spare.
Earlier on Wednesday, police and fire minister Nick Hurd said other tower blocks undergoing similar renovation work to Grenfell Tower will have emergency checks.
The blaze is likely to be deadlier than a 2009 fire in the city's south, which killed six people, including three children.
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