The number of people believed to have been killed in the Grenfell Tower fire in west London is expected to increase from 58, the Met police has said.
Commander Stuart Cundy said an updated figure for those presumed dead would be released on Monday.
But he said the process of identifying victims was likely to take weeks due to the conditions in the tower, which "verge on the indescribable".
A minute's silence is to be held later in memory of the victims.
The BBC understands about 70 may have died. Eighteen people remain in hospital, nine in critical care.
But Cdr Cundy warned of the potential "terrible reality" that some people may never be identified due to the intensity of the fire.
"I must consider the fact that there may be others in the building who, for whatever reason have not been reported to us.
"There is also a real possibility that there may be people in the building that no-one knows are missing," he said.
Police have released images from inside the 24-storey building to show the scale of the challenge they face.
The building was gutted after the fire ripped through the block in North Kensington in the early hours of Wednesday.
Cdr Cundy said: "It is really important that we are clear about the scale of the challenge facing us as our teams search Grenfell Tower to recover those people still inside and return them to their loved ones.
"Whilst our teams have been from the bottom to the top of the tower, we must now carry out a full forensic and systematic search.
"The conditions due to the fire damage verge on indescribable, which is why this will be such a lengthy operation taking weeks to complete."
A minute's silence is to be held at 11:00 BST across all government buildings to remember the people who lost their lives and all those affected by the fire.
Kensington and Chelsea Council has faced widespread criticism for its handling of the disaster, with residents complaining that officials had provided little support or information.
Prime Minister Theresa May has also come in for criticism over her response to the disaster.
She was jeered on a visit to the North Kensington estate on Friday, and protesters marching on Friday and Saturday called for her resignation.
London Mayor Sadiq Khan said the fire was a "preventable accident" caused by "years of neglect" by the local council and successive governments.
Kensington and Chelsea Council leader Nicholas Paget-Brown said he understood residents' anger and that the authority itself wanted to know why the fire had started and spread so quickly.
He added that the disaster was too big for one authority to handle alone and it was inaccurate to suggest his council was not present on the ground or working with other authorities.
The government has sent in a team of civil servants to bolster the relief effort.
They were spotted in high-visibility jackets in the area on Sunday afternoon.
The government has also pledged to give £5,500 to all of those who lost their homes.
Each household will receive at least £500 in cash and £5,000 paid into an account as part of a £5m emergency fund first announced on Friday.
Details of how the government's £5m emergency fund have been outlined, including:
- Funding will be made available for people staying in temporary accommodation
- A discretionary fund is available to help meet funeral costs
- There will also be funding for legal representation for residents involved in the public inquiry
- An extra £1.5m will pay for mental health support for the emergency services
A newly-established "Grenfell Fire Response Team" has been set up to lead the relief effort, which will include a 24-hour operation at the Westway Sports Centre.
The new team is made up of local and central government, the Red Cross, the Metropolitan Police and the London Fire Brigade.
Questions continue to be asked about why the fire spread so quickly, amid suggestions new cladding fitted during a recent overhaul could have been to blame.
On Sunday Chancellor Philip Hammond told BBC's Andrew Marr Show that a criminal investigation would examine whether building regulations had been breached when the block was refurbished.
Labour leader Jeremy Corbyn said that the council had seemed to "lack the resources to deal with a crisis of this magnitude", despite being the country's "wealthiest borough".
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