On Sunday, Turkey referred to Russia’s invasion of Ukraine as a “war,” signaling a change in vocabulary that might open the way for the NATO member nation to establish an international treaty restricting Russian naval access to the Black Sea.

Turkey holds authority over the Dardanelles and Bosphorus straits, which connect the Mediterranean and Black seas, under the 1936 Montreux Convention, and can block the passage of warships during warfare or if threatened.

Ankara has stated the Russian strike is unacceptable, but had not labeled the situation as a war until Sunday, in order to balance its Western commitments and tight ties with Moscow.

“On the fourth day of the Ukraine war, we repeat President (Tayyip) Erdogan’s call for an immediate halt of Russian attacks and the start of ceasefire negotiations,” presidential spokesperson Ibrahim Kalin said on Twitter.

Fahrettin Altun, Turkey’s communications director, said “we are witnessing yet another war in our region”, and repeated Erdogan’s offer to mediate.

Kyiv has appealed to Ankara to block any more Russian warships from passing toward the Black Sea, from which Moscow has launched one of its incursions on Ukraine’s southern coast.

But Turkey’s foreign minister said on Friday that Russia had the right under Montreux to return ships to their home base, which could limit any Turkish policy shift.

Turkey has cultivated good ties with both Russia and Ukraine. Any step too far against Moscow could harm its heavy energy and commodity imports and its tourism sector at a time of domestic economic turmoil.