LONDON (AP) — Fire swept through a high-rise apartment building in west London early Wednesday, killing an unknown number of people and sending more than 50 people to area hospitals.

Flames shot from windows all the way up the 24-story Grenfell Tower in North Kensington as more than 200 firefighters battled the blaze. A plume of smoke stretched for miles (kilometers) over the capital.

“This is an unprecedented incident,” Fire Commissioner Dany Cotton told reporters on the scene. “In my 29 years of being a firefighter I have never, ever seen anything of this scale.”

Cotton said a number of people died in the fire, but she couldn’t confirm how many because of the size and complexity of the building. Witnesses said that they saw people jumping from the upper floors.

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

The cause of the fire was not immediately known. Residents said the blaze appeared to start in an apartment on a lower floor and spread upward quickly.

People at the scene spoke of being unable to reach friends and family inside. Others said they could see people inside using flashlights and mobile phones to try to signal for help from higher floors.

Nassima Boutrig, who lives opposite 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. It was fire up, up, up. They couldn’t stop the fire.”