Evacuations are underway after a massive fire engulfed a 27-story apartment block in West London.

It's not clear how many people remain trapped inside the building or how many escaped, but witnesses say they saw people jumping from the tower.

"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.

Around 200 firefighters and 40 fire trucks have been deployed to tackle the blaze, as rescue workers try to evacuate the building in Lancaster West Estate in North Kensington, London Fire Brigade said. More than 20 ambulance crews and a hazardous area response team have also been dispatched.

The fire broke out before daybreak local time Wednesday, when most of the building's residents would have still been in bed.

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

In a statement posted to Twitter, London police say two people are currently being treated for smoke inhalation and they're waiting updates on further injuries.

"The whole building is on fire (and) spreading fast," witness Goran Karimi told CNN.

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 said it was a "major incident" on Twitter. (CNN)

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