A three-kilometre wall of flames up to 30 metres high forced hundreds of tourists into the sea off northern Greece, as wildfires also tore across, Croatia, France and the UK amid a punishing summer heatwave.

More than 200 firefighters, water-bombing planes and helicopters battled for hours to stop a blaze spreading across the Halkidiki peninsula, south of Thessaloniki, as it bore down on the resorts of Siviri and Fourka, packed with summer tourists.

Judging evacuation by road too dangerous, officials sent around 500 tourists into small boats, coast guard vessels and fire brigade craft, including roughly 200 taken to a ship waiting offshore, while cars trying to flee by road became gridlocked.

"We are on the beach and looking for a way to get away," one local homeowner, Sophia, told AFP, saying flames had already reached the town's houses.

Two firefighters were hospitalised, and authorities banned public access to the region's pine forests as forecasters warned of winds up to 100 km/h.