(CNN) -- More than 700 firefighters are battling a dozen forest blazes raging in northern Portugal, government agencies said Thursday.
Only one fire is considered under control and three firefighters have died so far.
Despite adverse weather conditions -- high temperatures of 40 degrees-plus Celsius (104 degrees Fahrenheit) and low humidity -- the government will not "stop fighting" said a statement from Rui Pereira, minister of Home Affairs.
The cause of the fires has not yet be determined but police are conducting an investigation. Portuguese police told CNN that 13 people have been arrested in connection to the fires.
Almost 9,000 forest fires have burned in Portugal this year, devastating about 19,000 hectares (48,000 acres).
Last year, 20 percent of Portugal's forest fires resulted from criminal activity while 70 percent were due to human negligence, according to the Forests National Authority.
CNN's Felipe Araujo contributed to this report.