Five thousand firefighters are struggling to contain multiple wildfires raging across northern Portugal that are blamed for causing at least six deaths and forcing an unknown number of residents to flee their homes.

Thousands of firefighters tackling deadly wildfires in central and northern Portugal had largely doused the flames in the Aveiro district, one of the worst-hit, as of Thursday morning, and were focusing on a dozen blazes still raging elsewhere.

More than 5,000 firefighters have been tackling the wildfires that Portuguese Prime Minister Luís Montenegro said are "raging across the country".