At least 64 people have died and over 150 injured in the blazes which broke out in the central Pedrogao Grande region over the weekend and spread quickly to other areas.
More than 1,000 firefighters and almost 400 fire engines are still battling to control the flames.
An official at Portugal's Air Accident Office said a Canadair water-dropping plane had crashed.
The aircraft reportedly came down near the village of Ouzenda while fighting the blazes, which have already destroyed more than 64,000 acres of forest.
A reporter said he heard a loud explosion and there had been thick smoke over the affected area.
He said a woman told him he saw the plane crash in a fireball.
But a spokesman for Portugal's civil protection agency said he could not confirm a Canadair aircraft had crashed.
Vitor Vaz Pinto said all 13 planes hired by the agency to help fight the blazes were accounted for on Tuesday evening.
However, 30 water-dropping aircraft were engaged in battling the blazes, some operating under bilateral agreements with the Portuguese government and others as part of an EU cooperation agreement.
Planes from Spain, France, Italy and Morocco were among those helping to fight the fires.
Officials said although the major wildfire in Pedrogao Grande was raging for a third straight day, it was mostly contained but still burning fiercely.
However, some resources were being diverted to Gois, about 12 miles away.
Commander Vitor Vaz Pinto said the Gois blaze was "very fast and very explosive" and had forced the evacuation of 11 hill villages.
Of the 64 victims, 47 died on one particular road - the N236 highway - including 30 who burned to death in their cars after being trapped by the flames.
It has been dubbed the "road of death" as Portugal's Prime Minister Antonio Costa led calls to find out why it had not been shut.
A survivor told Portuguese television that police directed them to the N236 as an alternative to the nearby IC8 route which had been closed and which officers used themselves.
"When we arrived at the IC8, they told us we couldn't pass and directed us towards the N236. We thought that the road was safe but it wasn't," said Maria de Fatima.
"We couldn't see anything, we couldn't even see the road, just the flames and the pine trees falling on the road."