"We are most likely facing the biggest tragedy of human life that we have known", Costa tweeted.
Image: Villagers sit outdoors as a forest fire burns near the village of Fato, central Portugal, June 18, 2017.
Dozens of fire engines drove back and forth to fight the raging blaze in areas as far as 20km north of Pedrogao Grande.
LISBON, Portugal (AP) More than 1,500 firefighters in Portugal are still battling to control major wildfires in the central region of the country, where one blaze killed 62 people.
Portugal was sweltering under a severe heatwave Saturday, with temperatures exceeding 40 degrees Celsius (104 degrees Fahrenheit) in several regions.
Interior Ministry official Jorge Gomes said that the firefighters had difficulty extinguishing the flames, which was "very intense" on at two out of the four locations where the fire spread.
He said at least 30 people died inside their cars as they tried to flee between the towns of Figueiro dos Vinhos and Castanheira de Pera. The prime minister said the death toll could rise as firefighters inspected charred remains of some buildings in remote villages.More news: 11 killed, 20 injured in Afghan Taliban attack
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Other bodies were found in houses in isolated areas.
Some 60 forest fires broke out across the country during the night, with around 1,700 firefighters battling to put them out.
Reinforcements, including more water-dropping planes from Spain, France and Italy, were due to arrive as part of a European Union cooperation program, officials said.
"So many deaths represent an nearly unprecedented tragedy in the history of democratic Portugal, " President Marcelo Rebelo de Sousa said in a televised speech on Sunday night. A lightning spark is believed to have started the fire on Saturday. The head of the national judicial police told Portuguese media that investigators have discovered the tree that was hit during the "dry thunderstorm".
As a result, a Spanish firefighting aircraft assisted yesterday morning with another expected. Portugal's president says the country's pain "knows no end" as it mourns at least 61 people killed in the deadliest wildfire in memory.
The Portuguese government has declared a state of emergency in the fire area, and also a three-day national mourning from Sunday and Tuesday.
UN Secretary-General Antonio Guterres on Sunday expressed his condolences to the Portuguese government and people.