By Joshua Berlinger, James Griffiths and Angela Dewan

CNN

LONDON (CNN) -- At least six people are dead and dozens injured after a massive fire rapidly tore through a 24-story apartment building in west London in the early hours of Wednesday, police said.

Fifty people were hospitalized as rescuers frantically worked through the morning to reach people believed still trapped in the Grenfell Tower in North Kensington.

Residents had reported concerns about fire safety in recent years, and witnesses said no alarm was sounded as the blaze took hold.

Latest developments

-- Residents say they were told to stay in their apartments. -- The fire is under control, but the the building is still smoldering. -- Authorities are trying to determine how many people are still trapped. -- 125 families live in the building, which also has a children's nursery.

Witnesses described people leaping from the building and of trapped children banging on windows as the fire broke out at around 1 a.m. local time.

A website run by the "Grenfell Action Group" said residents of the tower had expressed concerns over the safety of the building, specifically pointing to fire risks. Fire chiefs said it was too early to speculate on the cause of the blaze.

London Mayor Sadiq Khan declared the fire a "major incident." Around 200 firefighters, 40 fire trucks and 20 ambulance crews were at the scene.

"I can confirm six fatalities at this time but this figure is likely to rise during what will be a complex recovery operation over a number of days. Many others are receiving medical care," London Metropolitan Police Commander Stuart Cundy said.

The company that manages Grenfell Tower, Kensington and Chelsea Tenant Management Organisation described the fire as "devastating."

"Currently, we're focusing on helping those residents and London Fire Brigade is investigating the safety of the tower's structure but we will issue a further statement in due course," said Robert Black, the company's chief executive.

Leaping from windows

According to witness accounts, some residents were told to stay inside their apartments as the fire raged.

One woman said her friend inside for three hours and was told by police to wait and put towels down to block the doors. When no one came to help, she decided to escape on her own, the woman said.

Another witness said a family friend was "stuck on the eighth floor with her 5-year-old daughter" until 5 a.m., almost four hours after the fire broke out.

Turufat Yilma, who managed to escape, told CNN there was "no fire alarm at all." She only learned of the fire when a neighbor called her.

Two women who live nearby watched the blaze break out described the horror of seeing people leap from the tower to save themselves.

"They literally just jumped ... (they) must have thought, we're not going to sit here and suffocate," Samira Awil said, adding she had seen bodies of "kids, women, men" covered in sheets outside the building.

Tamara Eastmond said a lot of people appeared to be unaccounted for. "We literally watched a man burn to death in his flat," she told CNN. "We saw the flames enter his flat and (overcome) him."

Tia Abrahams, who lives close to the scene, got there before the fire service.

"There were people banging on windows screaming, crying out for help. There were even young children banging on the windows," she told CNN, adding she could hear screams as the hours went by.

Other witnesses said they saw residents holding their children out of windows.

Michael Paramasivan said he was watching TV in the building as his girlfriend and daughter slept when he smelled burning plastic. Soon he saw the smoke and chaos.

"I grabbed my little girl and ran down the stairs," he said. "Half of the building was ablaze by the time we got out. And it was just spreading like wildfire."

Grenfell Tower

Grenfell Tower is part of the sprawling Lancaster West Estate, an enclave of social housing managed privately on behalf of the London Borough of Kensignton and Chelsea. While the borough is one of the most affluent in London, North Kensington is a particularly deprived district.

The 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. The web page detailing the refurbishment was unavailable for a time on Wednesday.

Local councilor Robert Atkinson told CNN that the renovation works had modernized the building. "The building needed to be brought up to date with new kitchens and new heating systems," he said, adding that safety certificates had been issued and the council had been "told it had all been brought up to date."

Cladding had been added to the outside of the building, blamed by some residents for helping the fire to spread so quickly.

Investigation in 'early stages'

The blaze is likely to be deadlier than a 2009 fire in the city's south, which killed six people, including three children.

It was not yet possible to confirm the cause of the fire, which spread throughout the building from lower floors, London Fire Brigade Commissioner Dany Cotton said.

"In my 29 years of being a firefighter, I have never seen anything of this scale," she said.

She said the first fire trucks were on the scene within six minutes. "Crews wearing breathing apparatus ... have been working in extremely challenging conditions," she said.

Khan, the mayor, said that safety concerns and questions raised by the tenants will "need to be answered". He told the BBC that a situation where "people's safety is put at risk by bad advice or poor maintenance" could not be tolerated.

CNN's Steve George, Schams Elwazer, Vasco Cotovio and Karen Smith contributed to this report