A raging inferno engulfed a 27-story apartment building in London early Wednesday, sending giant flames and plumes of smoke billowing into the night sky and raising fears of many casualties.

Some 200 firefighters battled the blaze for at least five hours and more than 50 people were taken to hospitals with undisclosed injuries, according to the London Ambulance Service.

The BBC reported early Wednesday that “a significant number of people” were unaccounted for.

Fire Commissioner Dany Cotton called it an “unprecedented incident” unlike anything she had seen in her 29-year career as a firefighter. She said there were “a number” of fatalities.

Witnesses reported talking to trapped residents by telephone until the lines went dead, and others saw people standing at the windows, unable to make their way out, The Guardian reported.

Resident Siar Naqshabandi told The Guardian that residents were told “not to leave the building” and that he heard no fire alarms.

Some residents were seen using flash lights and cell phones to signal firefighters from their windows.

Assistant Commissioner Dan Daly put out a statement describing the difficulty of battling the blaze.

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

It was unclear what started the fire at about 1 a.m. local time. Flames were reported from the second floor to the top of the tower.

Witness Tim Dowine told The Guardian, “The sheer scale and the speed with which it spread, the closer you got, it seems like there must be casualties and fatalities.”

Witnesses claimed they could feel the heat from the flames from several blocks away.

“I’m 100 meters away and I’m absolutely covered in ash,” TV host George Clarke told Radio 5 Live.

“It’s so heartbreaking, I’ve seen someone flashing their torches [flashlights] at the top level and they obviously can’t get out,” he said.

Additional reporting by Joe Tacopino