The firefighters were in a vehicle that plunged into a ravine while they were battling the blaze in M'diq-Fnideq province late Monday, the authorities stated.
Four men suspected of starting the fire were arrested, they said.
Firefighting services, including Canadair water bombers, were being used on Tuesday to try to bring the flames under control and prevent them from spreading to populated areas.
The fire has so far destroyed about 90 hectares (about 220 acres) of forest land, according to initial estimates.
Morocco has been gripped by scorching temperatures, intense drought and stress on water supplies.
The agriculture ministry said more than 10,000 hectares of land in the country's north had been hit by forest fires this summer.
Last month forest fires in the north of the kingdom killed four people.
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