Hundreds of thousands of people have been relocated in both countries because of those fires, the latest natural disasters linked to sustained dry spells and successive heatwaves that many scientists say have been exacerbated by climate change.
"The fires that are hitting our country have reached a level that has never been seen before," French Prime Minister Sebastien Lecornu wrote on X.
So far this July, high temperatures have averaged 32.2C in the region of France hit by the fires, which is 7.3C above the month's average high between 1961 and 1990, according to data on Reuters Climate Monitor.
"We expect a fight that will be long and very difficult," Interior Minister Laurent Nunez later told reporters.
France's armed forces have been drafted in to help overwhelmed fire brigades.
On Saturday afternoon an A400M military transport aircraft, the biggest in France's fleet, joined the effort by dumping liquid over a flame-hit forest, Reuters TV footage filmed from a helicopter showed.
There, as in Spain, smoke billowed from the advancing wall of flames, leaving charred homes in its wake.
Three blazes in the vicinity of Madrid, two of which merged on Friday, have prompted Spain to declare its first national emergency over a wildfire.
A far smaller brush fire outside coastal Valencia, which began and was extinguished on Saturday, caused the death of an elderly man in his car, the interior ministry said.
Spain's Civil Protection Service said it expected four more water-dumping aircraft to arrive on Saturday, two from Italy and two from Greece - sent as part of a European Union response.
Firefighting aircraft from the Netherlands and Portugal have already been deployed in Spain.
High winds have stoked and spread the fires in both countries.
The coast near Bordeaux is a tourist hotspot which includes pine forests that can catch fire easily.
The city is also famous for its eponymous wines from nearby vineyards.
About 197,000 people have fled the Gironde and Landes departments close to Bordeaux since the wildfires broke out there this week.
Roughly 88,000 people have been relocated or confined to their homes in Spain because of the three major fires.
French Interior Minister Nunez said the flames were about 30km from Bordeaux.
The government has warned against road or rail travel to the area.
Bordeaux airport remained open but train and bus links to it were affected, it said on its website.
"I left without any of my things, with nothing, with the two little children", 71-year-old Bernadette said of her two grandsons after abandoning her house on the coast in the middle of the night.
She was speaking at a large conference centre in Bordeaux converted into emergency accommodation.
"I forgot plenty of things, my eye medications, everything."
Many Madrid residents have second homes in the area west of the capital where the two fires merged, which is still in the administrative region of Madrid.
A third fire in Avila province has threatened to merge with the two in Madrid.
Further complicating efforts to control them, one firefighters' union in Madrid is on strike, although the effect on numbers of firefighters deployed appeared small.
Spanish Health Minister Monica Garcia still warned against outdoor activities in Madrid because of the smoke.
"Avoid physical activity and staying outdoors. If you go outdoors and there is smoke or ash, use a mask," she said on X.
