Firefighters and emergency crews worked feverishly Wednesday to save lives and battle a massive fire that engulfed a 27-story building housing more than 100 apartments.

It’s not clear how many people remain trapped inside the building or how many escaped the inferno that began about 1:15 a.m local time, but witnesses say they saw people jumping from the tower in Lancaster West Estate in North Kensington.

“Chunks of the building are falling off in flames,” said CNN producer Salma Abdelaziz, who is outside the apartment block.

“It is a terrifying scene,” Abdelaziz said.

Latest developments

The fire broke out before daybreak, when most of the building’s residents would have been sleeping. 30 patients have been transported to five London hospitals, said London Ambulance Service. Around 200 firefighters and 40 fire trucks were deployed to the blaze, the London Fire Brigade said. More than 20 ambulance crews and a hazardous area response team were also dispatched. The flames have subsided significantly since the fire was extinguished at 5 a.m., but the building is still smoldering. The A40, a major road in West London, has been closed.

Stories of horror

Ayyube Assaf told CNN he saw the fire from his cousin’s house miles away and immediately ran toward the blaze. He and his cousin have family living on the 18th floor of Grenfell and wanted to make sure they were OK.

When they arrived, police wouldn’t let them go through. However, he eventually made his way inside.

“We brought them out and we held them in our arms, and the paramedics brought them to the hospital,” Assaf said. “You can’t get away from a fire on the 18th floor, but God’s grace, they got away.”

“Seeing your little cousin in your hands and not breathing properly, it’s a mad thing,” he said.

Kayo Mnemimeh saw lights flickering at the top of the building — something he thought was a signal from residents.

“Literally within five minutes of seeing that, that side of the building became engulfed in flames. I can’t say whether they got out or not,” he said.

He saw another man on about the 11th floor. The lights from his flat flickered too. Firefighters eventually shined their spotlight on his window.

“He came to the window, and there was fire all around where his flat was,” Mnemimeh.

Firefighters started spraying water cannons around him. Mnemimeh doesn’t think the crews could get to the stranded man at that time.

He was still trapped there when Mnemimeh spoke to CNN, and he’s not sure what happened to him.

Investigation in its early stages

The blaze looks to be the worst since a 2009 fire in south London killed six people, including three children.

“This is an absolutely devastating fire,” Nick Paget-Brown, the local council leader, told CNN. “By all accounts, it spread quickly.”

He told CNN it’s not yet clear how many people were evacuated or how many people are receiving support.

“We need more information. This is an early stage of an awful incident,” he said. “We have a clear emergency plan for dealing with the immediate incident and then there will need to be a thorough plan for helping residents who have lost their homes.”

In a statement posted to Twitter, London police said people are being treated for a range of injuries.

The London Fire Brigade said the cause of the fire is still unknown. In a statement posted on Facebook, the service said the “fire is from the second floor to the top floor of the 27 floor building.”

“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,” Assistant Commissioner Dan Daly said.

The sounds of sirens and helicopters is non-stop as locals and residents, evacuated from the building, begin to gather outside to watch the blaze.

London Mayor Sadiq Khan on Twitter called the fire a “major incident.”

Grenfell Tower

The 27-story Grenfell Tower was built in the 1970s and recently subject to a $10.9 million redevelopment, according to property firm Rydon. The building was to undergo specific improvements to fire safety and ventilation works.

Former fire marshal Robert Rowe told CNN the fire appears to have been “moving very quickly.”

He said he suspects the fire sprinkler system “was out of service or there was a malfunction at the time” based on how quickly the fire spread.

“What a fire sprinkler does, is stop it immediately, you won’t have a fire of an magnitude,” Rowe said. “It would stop it right there.”

Close to Notting Hill, the tower block is around a five minute walk from the Latimer Road Underground station and near the Westfield shopping center in the west London suburbs. It’s a mixed-development property that houses a boxing gym and a nursery, according to a news release last year from the Royal Borough of Kensington and Chelsea. Before the renovation there were 120 apartments in the building.

According to property website RightMove, the average rent in the building is around $2,500 a month.