They said the huge blaze remained "very voracious" with the work to put it out "very complicated".
Firefighters tweeted that they had deployed some 20 aerial units to help tackle the fire.
"Clearly the fire has not stabilised because it is still burning with great ferocity given that the weather conditions are almost like summer," Ximo Puig, leader of the Valencia region told Spain's RTVE public television.
The fire began just after midday (1200 GMT) on Thursday.
Firefighters said the blaze was more typical of summer than of late March.
With the vegetation dried out by a lack of moisture in the atmosphere in recent months and large amounts of combustible biomass in the forests, conditions were "perfect" for such a blaze, Manolo Nicolas of the Castellon firefighters had told public radio on Friday.
In 2022, which was a particularly bad year for wildfires in Europe, Spain was the worst-hit country with nearly 500 blazes that destroyed more than 300,000 hectares, according to figures from the European Forest Fire Information System (EFFIS).
