Wildfires are an annual problem in Portugal, where strong winds off the Atlantic blow into a hot and dry country.
More than 4,000 firefighters are battling 20 of the blazes, which are described as "major".
Spain's prime minister focused on criminal intent, and said authorities were certain the fires were caused by arsonists.
Meanwhile, Interior Minister Juan Ignacio Zoido also said that several people had already been identified in connection with the fires, and has appealed for anyone with further information to share it with the national protection service.
A wave of fires ravaged thousands of hectares of woodland in the northwestern Spanish region of Galicia, emergency service and media reports said.
He tweeted: "I'm heading to Galicia". Solidarity of all Spain and prompt coordination to combat the fire and assist people'.
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In a village in the commune of Vouzela, in the northern district of Viseu, residents used water hoses to try to fight the flames as several homes were consumed.
Agency spokeswoman Patricia Gaspar said the death toll could rise: "We are still searching burnt areas to see if there are any more victims".
Gaspar said the fires are likely to keep going due to unseasonably high temperatures of more than 30C (86F), although cooler, wetter weather is on the way. Two brothers in their 40s who were from her town and were trying to help put out the blaze were among the fatalities.
"We went through absolute hell, it was awful".
Five wildfires near Vigo, Galicia's biggest city, forced the evacuation of a shopping mall and a Peugeot Citroen vehicle factory on the outskirts of the city, though workers were able to return to the factory on Monday. There was fire everywhere, ' a Penacova resident told RTP television.
Two women died on Sunday after being engulfed in flames trapped in their van near Nigran, outside Vigo, and an elderly man died in an animal shed near his house in Carballeda de Avia.
The wildfires follow an enormous forest blaze in Portugal in June, which killed 64 people and injured more than 130.