Firefighters were struggling on Friday to control blazes in Marathon, northeast of the Greek capital, in Hania on Crete and on Chios in the Aegean Sea, amid strong winds and dry conditions.
All three blazes were in remote locations and were not a threat to residential areas, though strong winds were hampering the firefighting effort.
Greece's Civil Protection Authority, meanwhile, has advised residents and visitors to refrain from any outdoor activities that could spark a fire, such as barbecuing or burning plant cuttings and brush.
The areas most at risk on Friday are Attica, central and south Evia, the islands of the northern and eastern Aegean, southern Crete, the northeastern Peloponnese and parts of Thrace and eastern Macedonia.
The Meteo weather service also warned of treacherous sailing conditions at sea, particularly around the islands of the Cyclades and in the area south of the eastern Peloponnese.
Passengers planning to sail on a ferry boat on Friday and Saturday are advised to call their local port authority or ferry operator in advance to confirm their departure.