France has strongly backed Greece in a standoff with Turkey over natural gas resources and naval influence in the waters off their respective coasts.

The deal will see Greece buying 18 Rafale jets, 12 of them used, made by French firm Dassault to bolster its forces during their regular mid-air skirmishes with Turkish pilots over disputed Aegean airspace.

The deal "sends a clear message in several directions", said Greek Defence Minister Nikos Panagiotopoulos who oversaw a signing ceremony with French counterpart Florence Parly in Athens.

Greek government spokesman Christos Tarantilis said delivery of the first six planes would begin in July.

A group of Greek airforce pilots and technicians are to travel to France for training over the next few days, he added.

Parly's visit on Monday came on the day Greece and longterm regional rivals Turkey kicked off a round of exploratory talks over their clashing interests in the Mediterranean, their first in nearly five years.

The Greek government spokesman insisted the talks were "not negotiations" and were "not binding".

Greek Prime Minister Kyriakos Mitsotakis in September announced a "robust" upgrade to the country's armed forces.

The programme, Greece's most ambitious in decades, includes four multi-purpose frigates, four navy helicopters, anti-tank weapons, navy torpedoes, airforce missiles and 15,000 additional troops by 2025.

Turkey in August sent an exploration ship and a small navy flotilla to conduct seismic research in waters which Greece considers its own under postwar treaties.

Greece responded by shadowing the Turkish flotilla with its warships, and by staging naval exercises with several EU allies and the United Arab Emirates.

In contrast to other EU and NATO allies, France strongly backed Greece in the showdown with Turkey.