On a visit to a co-ordination centre for the massive firefighting effort in southwest France, Macron heard how the blaze raging since last week has been wildly unpredictable, transforming at times into a self-feeding fire storm as it burned through an area four times the size of Paris and forced 220,000 people to flee their homes.
"We're facing a completely unprecedented fire," Macron said, describing the situation as France's worst fire-related crisis since World War II.
He praised firefighting teams who stopped the inferno's progression on Monday after successive days of spreading flames.
But with woodlands still smouldering, he cautioned that the battle is not yet won.
"The weeks ahead will be hard," he said.
Interior Minister Laurent Nunez said earlier the fire had stopped growing but said caution was needed.
"It hasn't progressed during the night," Nunez said.
"We have to remain very concentrated, very determined, to go at any moment."
With thousands of firefighters on the ground and rotations of water- and retardant-dropping aircraft, it became a race against time on Monday before an expected heat wave that could make fighting flames more difficult.
The weather service forecast temperatures above 35C in the southwest on Tuesday and likely higher on Wednesday.
Regional authorities said the situation overnight to Monday had remained "globally stable" - a change from previous days when officials said flames had crept to within 15km of the outskirts of Bordeaux, a city of 268,000 people, and generated self-feeding firestorms with lightning bolts that ignited more blazes.
No new evacuations were announced on Monday.
But with France about to enter its peak August tourist season, authorities for the Gironde region around Bordeaux worked to keep holiday-makers away, banning holiday camps for children and for people with disabilities.
Crews using earth movers tore up strips of forest and scrubland to create barren spaces, intended to stop fires from spreading.
Farmers were keeping fire crews supplied with water, transporting it to them in tanks.
Around a dozen countries have provided planes, helicopters, firefighters and other assistance to both France and Spain, which is also fighting unprecedented blazes.
Fires in the two countries have forced more than 330,000 people to flee.
In Spain, wildfires have burned out of control around Madrid and in the Valencia region.
Authorities relocated 79,000 people and confined 30,000 others to their homes.
A fire raging in hilly, rural terrain in Avila, west of Madrid, has become Spain's largest on record, scorching 500 square kilometres, the government said.
"Look at how the valley is burned. Look at the houses, the people," said David Gonzalez, a 48-year-old gardener who used his van to run supplies to firefighters in Avila.
"We are feeling lot of anger and helplessness. It's something inhuman," he said.
