LISBON, July 26 (Xinhua) -- The sixth-generation fires -- the most extreme and uncontrollable category of forest fire -- are likely to become increasingly common as climate change intensifies, a leading Portuguese wildfire researcher warned on Sunday, citing the ongoing blazes in Spain and France as examples of the phenomenon.

Domingos Xavier Viegas, emeritus professor at the Faculty of Sciences and Technology of the University of Coimbra and one of Europe's leading wildfire specialists, told Portuguese news agency Lusa that the term "sixth-generation fire" refers to the enormous amount of energy released by such fires.

"When fires release more than 10 megawatts per meter, there is no capacity to suppress them through direct attack," he said, noting that in recent years fires have been recorded releasing 20, 30, 40 and even 60 megawatts per meter at their fronts.

At such intensity, the fires generate their own wind even in calm weather, making their behavior "very difficult to predict and control," Viegas said. Under these conditions, direct intervention at the fire front becomes impossible even with heavy aerial firefighting resources.

"The only option is to work on the tail and flanks, if conditions allow, or to create discontinuity zones using bulldozers. That is why we hear authorities saying their priority is to protect and evacuate people," he said.

Viegas said Portugal's catastrophic fires in 2017, which killed over a hundred people in two separate events in June and October that year, also qualify as sixth-generation fires based on the energy recorded at their fronts. ■