A new defence agreement between Greece and France will allow them to come to each other's aid in the event of an external threat, Greek Prime Minister Kyriakos Mitsotakis says, amid increased tensions with Turkey.
NATO allies Greece and France last month clinched a strategic military and defence cooperation pact, which includes an order for three French frigates worth about 3 billion euros ($A4.8 billion).
Athens had already ordered some 24 Dassault-made Rafale fighter jets this year.
"For the first time it is clearly stipulated that there be military assistance in the event of a third party attacking one of the two states," Mitsotakis told Greek lawmakers ahead of a parliamentary vote on Thursday to ratify the accord.
"And we all know who is threatening whom with a casus belli in the Mediterranean," he added, in a clear reference to Turkey.
Greece and Turkey are at odds over the extent of their continental shelves and their maritime boundaries, a fact which has hindered any expansion by Athens of its territorial waters to 19km in the Aegean.
Turkey, which lies to Greece's east, says such a move from Athens would be a casus belli, or cause for war. Some Greek islands lie less than 19km from Turkey's western coast.
All NATO allies are required to come to the aid of a member state that suffers aggression, but the decades-old tensions between Athens and Ankara are complicated by the fact that Turkey too is a member of the Atlantic alliance.
As well as the continental shelf issue, relations between Greece and Turkey have long been strained over other territorial issues in the Eastern Mediterranean, including air space, energy, the status of some Aegean islands, and the ethnically-split island of Cyprus.
The two countries re-launched exploratory contacts on their disputes earlier this year. Another round of bilateral discussions, which are informal, took place in Ankara on October 6.