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.

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

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.