Greece’s lawmakers were debating and voting Thursday on a broad five-year defense pact signed last week with France which includes a clause of mutual assistance in case of attack by a third party.

Prime Minister Kyriakos Mitsotakis signed the deal with French President Emmanuel Macron during a Sept. 28 visit to Paris, during which Greece also announced it would be buying three French frigates for the Greek navy.

The purchase and defense deal come at a time of generally increased tension between Greece and its fellow NATO member and neighbor Turkey over energy exploration rights in the eastern Mediterranean.

Thursday’s parliamentary debate and vote are for the defense pact only, not the purchase of the warships. The deal is expected to be ratified as the governing conservatives have a comfortable majority in parliament.

Describing the pact as “historic,” Mitsotakis said the deal meant the bolstering of Greece “in the troubled Mediterranean, and at the same time the first bold step in the strategic autonomy of Europe.”

The benefits of the deal, he said, were clear.

“For the first time, an explicit and unequivocal military assistance clause is provided in the case of a third party attack on one of the two states,” Mitsotakis said. “We all know ... who is threatening whom with a casus belli in the eastern Mediterranean.”

Turkey has said it would be a casus belli — a cause for war — if Greece extends its territorial waters from the current six to 12 nautical miles in the Aegean Sea between the two countries.

Turkey argues such a move would turn the Aegean into essentially a Greek sea. Athens maintains it has the right to extend to 12 nautical miles in accordance with the UN Convention on the Law of the Sea, and recently extended its western territorial waters, in the Ionian Sea facing Italy, to 12 nautical miles.

Regional rivals Greece and Turkey have been at loggerheads for decades over a series of disputes, including the delineation of the continental shelf, territorial rights in the Aegean Sea, aviation and maritime boundaries and the demarcation of Exclusive Economic Zones — areas where each country has exclusive rights to the exploitation of resources — in the Mediterranean.

Greece’s government announced last year it would be overhauling its military, including the hiring of personnel and a major military procurement program that has already seen the country buying 18 French Rafale fighter jets.

The defense deal with France includes a mutual assistance clause, which states that the two sides will come to each other’s aid “with all appropriate means at their disposal, and if necessary with the use of armed force, if they jointly ascertain that an armed attack is taking place against the territory of one of the two.”

The deal also includes a provision for Greek participation in French-led military operations such as those it has conducted in the Sahel region of Africa.

Mitsotakis said last week that the mutual assistance clause “essentially says that if any of the countries is attacked, if its territory is challenged, its sovereignty is challenged, then there is an obligation by the other party to assist it.”

The idea of collective defense is a principal tenet of NATO, of which both Greece and France are members, as is Turkey. Article 5 of the alliance’s treaty stipulates that an attack on one member nation is considered an attack on all.

“Does Article 5 apply in the case of an attack by a NATO member? I’m not sure NATO has ever been very clear on that issue,” Mitsotakis had said when asked during the conference why Greece needed an extra alliance agreement. “My obligation is to defend my country and to form the necessary alliances over and above the security arrangements that we already have.”

On Tuesday, NATO Secretary-General Jens Stoltenberg appeared to be critical of European defense initiatives that aren't within NATO.

“What I don’t believe in is efforts to try to do something outside the NATO framework or compete with or duplicate NATO, because NATO remains the cornerstone, the bedrock for European security, and also actually for North American security,” Stoltenberg said in a speech without directly mentioning the defense deal between Greece and France.