Firefighters in London responded early Wednesday to an “unprecedented incident” when a 24-floor residential apartment building became engulfed by flames, sending at least 30 to area hospitals and likely trapping residents inside.

London’s police force confirmed at least six dead. Authorities said they expect a number of fatalities and the BBC reported that a “significant” number of people were unaccounted for.

The building is the Grenfell Tower in the North Kensington area. The cause of the fire is not known. A structural engineer is monitoring stability of building.

One side of the building appeared to be in flames, and 45 fire engines and 200 firefighters were called to the scene. The London Fire Brigade tweeted that the fire involved the second to the top floor of the 27-story building.

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.”

Witnesses said that they saw people jumping from the upper floors.

The Metropolitan Police said 30 people were taken to five area hospitals. treated at the scene for smoke inhalation and cordons were in place. There has been no immediate word on whether anyone was still inside the building, but The Guardian reported that it appears—judging from the eyewitness accounts—there were likely people inside.

One witness, who identified herself as Sofia, told The Guardian that, “I heard loads of young girls crying out for help … I can’t see any ladders extending into the building … I can hear people screaming for help and they are dying.”

George Clarke, the presenter of “Amazing Spaces,” told Radio 5 Live he was covered in ash even though he was 100 yards from the scene.

He said he saw people waving flashlights from the top levels of the building and saw rescuers “doing an incredible job” trying to get people out.

London medics have treated 15 people for smoke inhalation. The London Ambulance Service said the 15 were taken to hospitals. Police said they were continuing to evacuate people from the apartment building Wednesday morning.

“Firefighters wearing breathing apparatus are working extremely hard in very difficult conditions to tackle this fire. This is a large and very serious incident and we have deployed numerous resources and specialist appliances,” an official said.

Tim Downie, who lives not far away, told Britain’s Press Association that he feared the building could collapse. He said he heard sirens, helicopters and shouting and then saw the building engulfed in flames.

“It’s the most terrifying thing I’ve ever seen. I just hope they have got everyone out,” he said. “People have been bringing water, clothes, anything they’ve got to help, out to the cordon.”