Experts said the building, which contained 138 flats, was covered with highly flammable cladding, which could account for the rapid spread of the blaze after it broke out on the fourth floor at around 5:30 pm on Thursday.
Dramatic images showed clouds of black smoke as the flames consumed the high rise of 138 flats in the Campanar district of the Mediterranean port city.
"Four people have died," Jorge Suárez Torres, deputy director of emergency services for the Valencia region, told reporters overnight.
"As of now, we have 14 people who remain untraced," regional administrator Pilar Bernabé added on Friday, stressing that the number could change.
Fifteen people were treated for injuries of varying degrees, including a seven-year-old child and seven firemen, but their lives were not in danger.
Fire crews on Friday entered the blackened ruin of the residential block, which stood dark against the crisp blue sky, its windows blown out and its once-white facade charred with the residue of smoke and flames, an AFP reporter at the scene saw.
Visiting the scene on Friday, Prime Minister Pedro Sánchez said the "priority now" is to search for the victims.
He encouraged people to "show empathy, affection and solidarity with the victims, with their families, with those who still do not know exactly what has happened" to their loved ones.
Smoke still wafted from the building though it was quickly blown away by strong gusts of chilly wind, which had fuelled the flames and complicated efforts to quench the blaze.
'A catastrophe'
Slava Honcharenko, a 31-year-old Ukrainian, said he knew several families of compatriots who lived in the building who had been relocated to a hotel since Thursday night.
"We feel very bad. We know what it is when you lose your house because we experienced this two years ago in Ukraine," he told AFP.
Spanish media said rescue workers had used drones to locate the bodies of those who perished.
Esther Puchades, deputy head of Valencia's Industrial Engineers Association (COGITI), told local media the fire had spread so rapidly because the building was covered with highly combustible polyurethane cladding.
The fire, which started in an intermediate floor, spread within minutes to the entire building, according to residents.
Sergio Pérez, a 49-year-old driver who lives nearby, said the building burned as if someone had "poured gasoline" on it. "It's a catastrophe. Unimaginable. It's devastating," he said.
Dramatic rescue
As the fire raged, residents could be seen waiting to be rescued on balconies.
Firefighters used a crane to pluck a father and his daughter from a balcony where they were trapped in an operation broadcast live on national TV. Onlookers cheered and applauded as they were brought to the ground.
Other dramatic footage showed a man jumping several floors onto an inflatable mat to escape the flames.
Valencia has announced three days of mourning and suspended the start of the month-long annual Fallas festival.
Fuastino Yanguas of the Valencia fire brigade said the material used on the facade of the building must be investigated as it "was a factor that contributed a lot" to the lightning spread of the flames, as were the strong winds, with gusts of up to 60 kilometres (40 miles) per hour at the time the blaze broke out.
The fears of polyurethane cladding exacerbating the Valencia fire recalled the 2017 tragedy at London's Grenfell Tower, when a fire killed 72 people in the 24-storey high rise.
The blaze spread rapidly because of the highly combustible cladding on the block's outside walls. A public inquiry into the disaster has yet to publish its final report.
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