"I am sad to confirm that we now know there have been fatalities and more than 50 people have been taken to hospitals", he said in a statement early Wednesday.
"This is a complex operation and I anticipate the number will increase beyond those 12".
Cundy said an extensive area was sealed off around the decimated building, which was engulfed in flames in most parts from the second floor up to the roof.
Residents who escaped spoke of others trapped and screaming for help, with some throwing children from windows and others jumping from upper floors.
Fire crews are continuing to tackle "pockets of fire" in the building with many people still unaccounted for.
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The fire brigade department issued a statement saying the fire had spread from the second floor to the top floor of the building. Police confirmed that 12 people died but they expect to toll to rise after a complex recovery operation that will last days.
At the present time, the London Ambulance Service have stated that they're treating 68 patients and took them to six hospitals across London, where 18 people are now in critical care. "Crews wearing breathing apparatus have been working tirelessly in extremely hard conditions to rescue people and to bring this major fire under control". "I'm standing here in everything I've got", one survivor said.
Prime Minister Theresa May's office said she was "deeply saddened by the tragic loss of life" in the fire.
Khan, the London mayor, said there needed to be answers after some residents said they had been advised they should stay in their flats in the event of a fire.
On her first full day of work, after being sworn in as an MP yesterday, she said she had visited several support centres set up for those affected by the blaze and would continue to do so throughout the day.
"There were people screaming "Help me".
The trigger for the fire has been attributed to a faulty fridge on one of the lower floors - the third or fourth floor - however some eyewitnesses also reported seeing blue flames which could indicate a gas explosion.
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He was admitted to McClaren Flint hospital in June 2015 with symptoms consistent with pneumonia. Wells have been and continue to be instrumental in Flint's recovery", he said in a statement.
Sandwell Council leader Steve Eling said: "The fire in London is an absolute tragedy and my thoughts are with those killed, injured and who have lost their homes".
It is unclear how many people were inside the building when the fire broke out, but the tower comprises around 130 flats and houses more than 200 people, according to United Kingdom media reports.
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Emergency checks will be carried out on tower blocks going through similar refurbishment to Grenfell Tower, the Government has said.
Witnesses said the fire spread rapidly and traumatic accounts have emerged of the desperate attempts made by residents to flee the flames.
He said: "The fire started happening on third floor, we called the fire bridge who came 20 minutes later, and then the whole thing just went up".
The cause of the fire has not yet been revealed, but residents told the AP they had repeatedly warned building administrators about a potential fire threat.
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Mike Doyle told ABC News. " He said, 'I understand, ' and said he needed to take the gun to a range to shoot it, Watson said". She said he was using a long gun and exchanging fire with law enforcement officers, one of whom yelled for her to get down.
One person could be seen looking out of a section of the building, just below a burnt-out floor, with a piece of clothing dangling from the window frame.