"Firefighting in high rise blocks can be incredibly challenging; crews train extensively to attend and respond to these type of incidents quickly, however they can be very complex".
One witness said she heard people screaming and could see residents flashing their phone lights inside the building. Some were evacuated in their pajamas.
The cause of the fire, which broke out just before 1 am (0000 GMT), is not known, the Fire Brigade said.
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"I sat there watching my house burn down and watching people cry for help who couldn't come down", Mamudu said.
Samira Lamrani, a witness, was quoted by The Telegraph as saying that she saw a woman try to save a baby by dropping it from a window "on the ninth or 10th floor" to waiting members of the public below.
David Collins of the Grenfell Tower residents association, told BBC News that residents concerns about fire safety had been ignored by the local council.
Fire swept through a high-rise apartment building in west London early Wednesday, killing an unknown number of people with around 50 people being taken to hospital.
A major incident has been declared with dozens of people being treated at hospitals across the captial.
"It's so heartbreaking, I've seen someone flashing their torches at the top level and they obviously can't get out".
Throughout the day, we've seen countless harrowing accounts from people that were at the horrific scene, but this resident has raised serious questions about the recent regeneration work that was done on the building, and other buildings in the area. Fire chiefs said it was too early to speculate on the cause of the blaze.
Mr Cundy added: "Whilst we may have accessed every floor that is not the same as a full search of the whole building, and as I said while we now sadly have 12 fatalities, I do believe that figure will rise and sadly I don't anticipate that there will be further survivors".
"It's the most terrifying thing I've ever seen".
"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". When she got downstairs there was only four of them with her.
A local residents association had previously warned it was anxious about the risk of a serious fire in the block.
Hurd has spoken to London Fire Brigade Assistant Commissioner Steve Apter to assure him that the government will offer assistance. Were fire safety regulations followed by the managers of the tower, Kensington and Chelsea Tenants Management Organisation (KCTMO), and how much oversight was there from the owners, the Royal Borough of Kensington and Chelsea council?