Television footage showed the building’s facade ablaze, with burning segments falling to the pavement and small explosions audible inside. Witnesses said the fire, fanned by strong winds, spread to the entire building in half an hour.
Residents could be seen calling for help from balconies. A fireman had to jump from the first floor to a mattress below.
Prime Minister Pedro Sanchez was on his way to the scene after expressing support for those affected and praising firefighters.
The fire began on Thursday evening on the fourth floor of the building in an affluent neighbourhood of Spain’s third-largest city and spread to other apartments, emergency services said.
Valencia’s mayor, Maria Jose Catala, told reporters on Friday that four people had been confirmed dead, and from nine to 15 were missing.
Jorge Suarez, deputy head of emergency services in the Valencia region, said there appeared no risk of the structure collapsing for now, but firefighters were operating from outside.
Tackling the fire inside was not immediately possible, and the firefighters were first trying to cool the facade, he said.
The building had 14 floors with hundreds of apartments, local media said.
Pilar Bernabe, government representative for the region of Valencia, said it was hard to say how many people were missing because it was “a building with many flats, flats in which there were people of foreign nationality, whose location is more difficult to pinpoint.”
Esther Punchades, a representative of insurance inspection agency APCAS, told state broadcaster TVE a lack of firewalls and the use of the plastic material polyurethane on the facade would have contributed to the rapid spread of the blaze.
The spread of a fatal fire in the Grenfell Tower block in London in 2017 after an electrical fault was blamed on the use of highly flammable external cladding.
The city has decreed three days of mourning and suspended the start of a month-long annual festival.
