Thousands of firefighters backed by soldiers and water-bombing aircraft on Monday battled over 20 major wildfires raging across western Spain, where officials say a record area of land has already been burnt.

Thousands of firefighters worked overnight to contain dozens of deadly wildfires burning across Spain and Portugal as heat and gusty winds spread blazes through dry brush, forests and crops.

This year is turning into one of the worst fire seasons in recent memory in Europe. Firefighters are struggling to contain out-of-control wildfires in western Spain and Portugal.

Spain's weather agency warned the public on Monday to be "cautious" of the "very high or extreme fire danger."

Wildfires have ravaged more than 450,000 hectares of land across the Iberian peninsula since the start of the year, setting records in a region particularly hard hit by intensive heatwaves and drought affecting southern Europe.

A heatwave is continuing to affect the Iberian Peninsula. Thousands of firefighters have been deployed to battle ongoing blazes and towns have been evacuated. Spanish Prime Minister Pedro Sánchez visited affected regions to meet with the heads of emergency teams.

Spain's worst wave of wildfires on record spread to the southern slopes of the Picos de Europa mountains on Monday and prompted authorities to close part of the popular Camino de Santiago pilgrimage route.

"The evolution is favourable, the teams are stabilising the fires," Castile and Leon regional leader Alfonso Fernandez Manueco told reporters. The region, Spain's largest, is one of the worst hit by the blazes.