A Spanish forest fire raged out of control on Thursday amid a European heatwave, devouring land as hundreds of firefighters battled through the night, local authorities said.

The blaze broke out Wednesday afternoon in Torre del Espanol in the northeastern region of Catalonia and by Thursday had destroyed more than 5,500 hectares (13,500 acres), the regional government said.

The fire could eventually devour 20,000 hectares in what was presented as an "extreme risk", a statement said.

Some 350 firefighters backed by around 230 soldiers and 15 aerial tanker aircraft were at the scene of the blaze, the worst in Catalonia in 20 years.

The fire spread quickly due to strong winds and soaring temperatures / © UME/AFP

The fire raged several kilometres from the Asco nuclear plant but officials said the site was not at risk since winds were blowing the flames away from it.

"The difficulties are such that we can't talk about a fire that is under control or in the extinction phase, but rather that we're at a moment when the blaze is getting bigger," regional interior minister Miquel Buch told Catalan radio.

Buch said it might have been caused by "an accumulation of manure in a farm that generated enough heat to explode and generate sparks."

Firefighters said that steep terrain was hampering efforts to control the blaze, which spread quickly due to strong winds and soaring temperatures which neared 40 degrees Celsius (104 Fahrenheit).

Many evacuees told Spanish media they fled with just the clothes on their backs / © AFP

"It's complicated. We won't get it stabilised today," regional firefighting chief Manuel Pardo told Spanish public television.

Around 50 people have been evacuated from their homes and five roads have been shut, the regional government said. Many evacuees told Spanish media they fled with just the clothes on their backs.

Emergency services ordered the roughly 250 residents of the village of Bovera to stay indoors.

The charred land includes vineyards, olive groves and fruit trees. A farm burned down at Torre del Espanol, killing more than 200 sheep and at least two horses, according to an AFP photographer at the scene.

Meteorologists blame a blast of hot air from northern Africa for scorching temperatures early in the European summer, which could send thermometers towards 40C in France, Spain and Greece on Friday.