Fire crews on Friday picked through a still-smouldering 14-storey apartment block in Valencia, eastern Spain, a day after a blaze ripped through the building killing at least 10 people.

Experts said the building 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 (1630 GMT) on Thursday.

Investigators have still to determine the cause of the fire.

Film footage showed clouds of black smoke as the flames consumed the high rise of 138 flats in the Campanar district of the Mediterranean port city.

On Friday afternoon, officials updated the death toll, which had previously stood at four.

"We can confirm that following a first inspection, forensic police have found 10 fatalities," said regional administrator Pilar Bernabe.

It was still not clear if other people were missing, but local officials have not ruled out the death toll rising.

Another 15 people were treated for injuries of varying degrees, including a seven-year-old child and seven firefighters, but their lives were not in danger.

Fire crews on Friday entered the blackened ruin of the residential block, its windows blown out and the once-white facade charred with the residue of smoke and flames.

Prime Minister Pedro Sanchez visited the scene.

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. (AFP)