LONDON -- They banged on windows, screamed for help, dropped children from smoky floors in a desperate attempt to save them. Terrified residents of the Grenfell Tower said there was little warning of the inferno that engulfed their high-rise apartment building and left 12 people dead -- a toll that officials said would almost certainly rise.

The blaze early Wednesday in the 24-story building in west London's North Kensington district also injured 74 others, 18 of them critically, and left an unknown number missing. A tenants' group had complained for years about the risk of a fire.

More than 200 firefighters worked through the night and were still finding pockets of fire inside later in the day. A huge plume of smoke wafted across the London skyline and left a burned-out hulk in the working class, multi-ethnic neighbourhood.

"In my 29 years of being a firefighter, I have never, ever seen anything of this scale," Fire Commissioner Dany Cotton said.

Up to 600 people lived in 120 apartments in the Grenfell Tower. After announcing the updated death toll of 12 in the afternoon, Cmdr. Stuart Cundy said that "we believe this number will sadly increase."

Crews rescued 65 people, said Steve Apter, the fire brigade's director of safety and assurance.

Prime Minister Theresa May's office said she was "deeply saddened by the tragic loss of life" in the fire.

"My thoughts are with the victims, their families and all of those who had their homes destroyed," she said. "It's impossible to comprehend the horror of what they've been going through."

London Mayor Sadiq Khan said many questions must be answered about safety for the scores of other apartment blocks around the British capital.

The London Fire Brigade said it received the first reports of the blaze at 12:54 a.m. and the first engines arrived within six minutes.

Survivors told of frantic attempts to escape during the nighttime fire. Some initially feared it was terrorism-related, although authorities have not suggested that terrorism was involved.

"The flames, I have never seen anything like it. It just reminded me of 9-11," said Muna Ali, 45. "The fire started on the upper floors. ... Oh my goodness, it spread so quickly. It had completely spread within half an hour."

Samira Lamrani said she saw a woman drop a baby from a window on the ninth or 10th floor to people on the sidewalk.

"People were starting to appear at the windows, frantically banging and screaming," Lamrani told Britain's Press Association news agency.

When the woman indicated she was going to drop the infant, "a gentleman ran forward and managed to grab the baby," she added.

Joe Walsh, 58, said he saw someone toss two children out a window on the fifth or sixth floor. Tiago Etienne, 17, said he saw about three children between the ages of 4 and 8 being dropped from around the 15th floor. There was no word on their fate.

Other residents told harrowing tales of their own escapes and frustration at not being able to help neighbours.

Ruks Mamudu, 69, said she ran to safety down one flight of stairs to the ground floor from her apartment wearing only her purple pyjamas and bathrobe. She and her grandson sat outside the building, helplessly watching those trapped on higher floors.

"I sat there watching my house burn down and watching people cry for help who couldn't come down," Mamudu said.

Nassima Boutrig, who lives across from the building, said she was awakened by sirens and smoke so thick that it filled her home as well.

"We saw the people screaming," she said. "A lot of people said, 'Help! Help! Help!' The fire brigade could only help downstairs. ... They couldn't stop the fire."

Resident Hamid Wahbi said that as he fled, he asked about a neighbour's father but was told he was still inside.

"We tried to go back, but it was all black, so I had to come out of the building," Wahbi added.

There was no immediate word on the cause of the blaze, but the Grenfell Action Group has been warning about the risk of fire at Grenfell Tower since 2013.

Edward Daffarn, who lived on the 16th floor, said the building's fire alarm didn't ring. He said residents had complained for years to Kensington and Chelsea Council about the building's safety, to no avail.

"I'm lucky to be alive. A neighbour's smoke alarm went off and another neighbour phoned and told me to get out," Daffarn said. "I consider this mass murder."

The Action Group expressed concern about the testing and maintenance of firefighting equipment and blocked emergency access to the site. In a Nov. 20 blog, the group predicted that only "a catastrophic event" leading to "serious loss of life" would bring the outside scrutiny needed to make conditions safe for residents.

"All our warnings fell on deaf ears and we predicted that a catastrophe like this was inevitable," the group said after the fire broke out.

The Kensington and Chelsea Council, which oversees the area where the fire occurred, said in a statement its immediate focus was helping victims and their families. It said the cause of the blaze would be "fully investigated."

Built in the 1970s, the housing block was recently upgraded at a cost of 10 million pounds ($12.8 million), with work finishing in May 2016, according to the local council. Rydon, the British company that did the refurbishing, said in a statement that its work "met all required building control, fire regulation and health and safety standards."

Britain's government ordered checks at tower blocks going through similar refurbishment amid concerns that renovations at the Grenfell Tower contributed to the spread of the blaze. It was not immediately known if the building had a sprinkler system.

Authorities will "seek to identify towers that might have a similar process of refurbishment, run a system of checks so that we can, as quickly as possible, give reassurance to people," said Policing and Fire Minister Nick Hurd.

The Grenfell tenants' organization's newsletter instructs residents to stay put in a fire unless the blaze is in their own apartment or in their hallway -- the same guidance used in multistory hotels and other high-rise buildings. The organization's July 2014 newsletter said Grenfell "was designed according to rigorous fire safety standards."

Neighbours began helping survivors with clothing, food and water as well as offering shelter.

Churches and a nearby mosque served as gathering points for donations for those who raced out of the burning building with little else than what they were wearing. Social media sites joined the effort, with some Londoners offering a space on their sofas for those affected by the blaze.

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Associated Press writers Cara Rubinsky, Sylvia Hui and Ben Jary contributed.

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This story has been corrected to show that the building has 24 floors, not 27.

Here is a sampling of messages, images and video of the Grenfell Tower fire being shared on social media:

Truly terrible scene this morning of the fire at #GrenfellTower, London. pic.twitter.com/qLXzGJri9Q — Jason Hawkes (@jasonhawkesphot) June 14, 2017

I really hope everyone got out ok. #PrayersForGrenfell #london #kensington #fire #GrenfellTowerFire pic.twitter.com/FHZ6cgMN9s

— Kevin W. (@kwilli1046) 14 June 2017