LONDON — A massive fire ripped through a 24-story apartment complex in west London early Wednesday, killing an unknown number of people, London's fire chief said. At least 50 people were sent to the hospital as a result of the blaze.

London Fire Commissioner Dany Cotton told reporters she could not confirm the number of deaths due to the complexity and size of the building. "This is an unprecedented incident. In my 29 years of being a firefighter I have never ever seen anything of this scale," she said.

Cotton said she had "no idea" how many people were unaccounted for and urged residents who escaped the blaze to report to authorities so they could be listed as safe. Witnesses said some residents were missing and reported seeing people jumping from their apartments to escape the blaze, according to the BBC.

The British broadcaster reported that 500 people lived in the apartment block, in the North Kensington area, which is near Notting Hill.

Most of the building was engulfed in flames. The cause of the fire wasn't immediately known. The building was being monitored by structural engineers amid concerns it could collapse. There are 120 apartments in the building. It was constructed in 1974.

London Fire Brigade said 200 firefighters responded to the fire in the Grenfell Tower complex soon before 1 a.m. local time.

"Our priority is to assess the level and nature of injuries and ensure those in the most need are treated first and taken to hospital," the London Ambulance Service service said. London Mayor Sadiq Khan tweeted that a major incident had been declared.

Forty fire engines were deployed to the scene and hours after the fire broke out, a plume of smoke was seen from miles away.

"I watched one person falling out, I watched another woman holding her baby out the window... hearing screams," Jody Martin, who was present during the chaos, told the BBC.

"I was yelling at everyone to get down and they were saying 'we can't leave our apartments, the smoke is too bad on the corridors.'"

Michael Paramaseevan, who lives on the seventh floor with his partner and young daughter, told the British broadcaster that he ignored official advice to stay in his apartment.

"If we had stayed in that flat, we would've perished. My gut instinct told me just to get the girls out. I wrapped the little one up because of the smoke and I just got them out," he said.

Witnesses who posted images on social media said that all the block's floors were filled with flames.

Residents were told to stay in their apartments in the event of fire, the Independent reported.

"Thankfully residents didn't take that advice but fled... these are some of the questions that have to be answered," Mayor Khan said, according to the newspaper.

The Grenfell Action Group, a group of residents of the building, said it previously warned Kensington and Chelsea council — the local authority which owns the building — and Kensington and Chelsea Tenant Management Organisation (KCTMO) — the company that manages the homes — that the building was a serious fire risk but those concerns were dismissed.

"The Grenfell Action Group believe that the KCTMO narrowly averted a major fire disaster at Grenfell Tower in 2013 when residents experienced a period of terrifying power surges that were subsequently found to have been caused by faulty wiring," the action group wrote in a blog post in November.

Kensington and Chelsea council and KCTMO did not immediately respond to a request for comment.

Contributing: Charles Ventura in Los Angeles. Eversley reported from New York.