Deadly wildfires in central and northern Portugal have pushed emergency services to the limit and much-needed reinforcements will arrive on Wednesday from Spain and Morocco, the civil protection authority said.

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.

Duarte Costa told CNN Portugal late on Tuesday that a specialised emergencies team of 230 Spanish military personnel would be deployed in the central Viseu district, where huge blazes are "of great concern at the moment".

Morocco is sending up to four heavy water-bombing aircraft that should also arrive in Portugal on Wednesday, he added.

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, and that is why we are asking for help from the European mechanism, Spain and Morocco," Costa said, adding that the reinforcements would allow for some 
rotation of exhausted Portuguese firefighters and aircraft maintenance.

The government has declared a state of calamity in all municipalities affected by the wildfires, allowing civil protection agents to access private property.

Prime Minister Luis Montenegro said a government team would coordinate the provision of "the most immediate and urgent support" to those who have lost their homes and livelihoods.

At least some of the dozens of fires across Portugal have been caused by arsonists, prompted by possible commercial interest, spite or criminal negligence, he said, vowing to "spare no effort in repressive action" against such crimes.

Portugal's national guard, or GNR, said in a statement they had arrested seven people since Saturday suspected of arson in the districts of Leiria, Castelo Branco, Porto and Braga.