A wave of deadly wildfires in Portugal has pushed emergency services to the limit and much-needed reinforcements arrived on Wednesday from Spain and Morocco, authorities said, as cooler weather also brought a glimmer of hope.

At least seven people have died due to the blazes in the Aveiro and Viseu districts, with dozens of houses destroyed and tens of thousands of hectares of forest and scrubland consumed. Authorities have mobilised more than 5,000 firefighters.

Reuters footage showed local residents in the northwestern Aveiro district - one of the worst-hit - distributing food and water to exhausted fire engine crews, wishing them "strength in the combat".

Chicken farmer Paulo Fernandes and his neighbour struggled to contain the flames around his property in Sao Pedro do Sul in the adjacent Viseu district with a hosepipe.

With firefighters nowhere to be seen, he sounded stoic: "They didn't come here, they are spread out in the area and they can’t be everywhere, you know... The planes could come here yes, but we have no say in that so we have to protect our property."

A 270-strong Spanish military emergencies team with bulldozers was deployed in the same Viseu district and two heavy water-bombing aircraft arrived from Morocco, with two more on the way, civil protection authority said.

Spain, Italy and France have already sent two water-bombing aircraft each after the Portuguese government on Monday requested help under the EU civil protection mechanism.

"We are in a stressful situation, at the limit of our capabilities," civil protection chief Duarte Costa told CNN Portugal, adding that the reinforcements would allow for some rotation of exhausted Portuguese firefighters and aircraft maintenance.

Data from the European Forest Fire Information Service showed that large-scale blazes had burned an area of more than 90,000 hectares (347 square miles) since Saturday, making this year's total of at least 124,000 ha the widest burned area since 2017, when Portugal suffered two devastating wildfires that killed more than 100 people.

After temperatures that had kept above 30 Celsius (86 Fahrenheit) since the weekend, officials expected cooler air to help with the firefighting effort in the coming days, but the meteorological situation in general remained unfavourable, civil protection commander Andre Fernandes told reporters.

At least some of the dozens of fires across Portugal have been started by arsonists, motivated by possible commercial interest, spite or criminal negligence, police said, and Prime Minister Luis Montenegro vowed to "spare no effort in repressive action" against such crimes.

Police said in a statement they had arrested 12 people since Saturday suspected of arson in the districts of Aveiro, Leiria, Castelo Branco, Porto and Braga.