A massive fire, which has engulfed a high-rise apartment block in West London, has left "a number" of people dead and several others injured, fire officials say.

The fire broke out in the 27-storey Lancaster West Estate tower in the early hours of Wednesday when most of the residents would have still been in bed.

The fire brigade said 45 fire engines and 200 firefighters as well as 20 ambulances were sent to the building which has 120 flats and was built in 1974, but recently underwent a major refurbishment.

"Firefighters wearing breathing apparatus are working extremely hard in very difficult conditions to tackle this fire," London Fire Brigade assistant commissioner Dan Daly said.

"This is a large and very serious incident and we have deployed numerous resources and specialist appliances," he said.

Several people are trapped inside the building with reports that it might collapse. Mayor of London Sadiq Khan said a "major incident" had been declared.

A witness told CNN that he saw people jumping from the building.

Police officials said that they had started an evacuation of the block and the neighboring buildings, adding some people were being treated at the scene for smoke inhalation.

At least 50 people have been taken to hospital so far, according to officials.

“We have now taken over 50 patients to five hospitals across London,” London Ambulance Service Assistant Director of Operations, Stuart Crichton, said.

London Fire Commissioner Dany Cotton said, "I am very sad to confirm that there have been a number of fatalities."

"I cannot confirm the number at this time due to the size and complexity of this building and it would clearly be wrong for me to speculate further," Cotton added. Equally, the cause of this fire is not known at this stage."

The streets around the tower have been sealed off and there was chaos there with "people in night clothes, no shoes, crying, looking for family.”

George Clarke, the presenter of Channel 4 TV program Amazing Spaces, told Radio 5 Live, "I';;;m 100 meters away and I';;;m absolutely covered in ash."

"It';;;s so heartbreaking, I';;;ve seen someone flashing their torches at the top level and they obviously can';;;t get out."

"People can be seen at the windows, including one man waving a blanket from the window," a witness told The Guardian.

"Some shouting “put your head out the window” or calling the man with the blanket to shout his flat numbers so they can guide the fire services to him."

Hanan Wahabi, 39, told AFP she escaped with her husband and son, 16, and daughter, 8, but feared for her brother and his family who live on the 21st floor.

“Last time I saw him they were waving out the window, his wife and children,” she said, sitting outside the Rugby Portobello community centre.

“I’ve not heard from them since, the phone is not going through, the landline isn’t going through. That was about 2:00 am (0100 GMT), she said.

Eddie, 55, who lives on the 16th floor, said he ran out of his house with a wet towel wrapped around his head as smoke engulfed the flat.

“You couldn’t see anything. I just ran down the stairs… Loads of people haven’t got out of the building,” he said.

Another witness named as Jody Martin told the BBC that he battled his way his way to the second floor only to encounter choking smoke.

“I watched one person falling out, I watched another woman holding her baby out the window… hearing screams, I was yelling everyone to get down and they were saying ‘We can’t leave our apartments, the smoke is too bad on the corridors’,” he said.

By early morning, most of the block was a smouldering hulk as firefighters sprayed water onto floors within reach of appliances on the ground.

Frantic families at the scene attempted to call their loved ones, fearing they could be stuck inside, and were being directed by police to a nearby restaurant where some of the injured were being treated.

The fire brigade said 40 fire engines and 200 firefighters had been called to the blaze at Grenfell Tower, which has 120 flats.

“Fire is from second to top floor of 27 storey building,” the fire service said on Twitter. Fire-fighters said they had managed to evacuate residents up to the 11th floor.

“We can confirm that we have taken 30 patients to five London hospitals following the incident,” said Stuart Crichton, assistant director of operations at the London Ambulance Service.

He added that more than 20 ambulance crews as well as a “hazardous area response team” were at the scene.

“Our priority is to assess the level and nature of injures and ensure those in the most need are treated first and taken to hospital.”

Police cleared nearby buildings because of fears about falling debris and shut down a section of the A40 highway — a normally busy thoroughfare into London.

A London Underground line passing the area near Latimer Road station was also shut down. “We’ve set up emergency centres nearby and people have been evacuated to those,” local councillor Nick Paget-Brown said on Sky News.

“It’s clearly been a devastating fire,” he said. “There’s a lot more work to do to evacuate the building and to establish how safe it is.”

The apartment block was built in 1974, but had recently undergone a major refurbishment, including a new heating and hot water system.

The refurbishment cost £8.7 million (9.9 million euros, $11 million) and was completed in May 2016. The exterior was modernised with cladding.

Local residents had warned a year ago about a potential fire risk caused by rubbish being allowed to accumulate during improvement works.

“This matter is of particular concern as there is only one entry and exit to Grenfell Tower during the improvement works,” read a blog post by the Grenfell Action Group.

“The potential for a fire to break out in the communal area on the walkway does not bear thinking about as residents would be trapped in the building with no way out,” it said.