Ankara had called the Russian invasion a "military operation", but Foreign Minister Mevlut Cavusoglu said: "It's no longer a military operation but a state of war," in an interview with CNN Turk television.

"In this case, of course, whichever of them, it can be Russia or Ukraine, we apply the Montreux agreement," he added.

The 1936 Montreux Convention governs the free movement of commercial ships in peacetime through the Bosphorus and the Dardanelles straits and grants Turkey the right to block warships in wartime if threatened.

Ukraine has officially asked Turkey to close the Dardanelles strait -- and thus access to the Black Sea -- to Russian ships.

NATO member Turkey, which has strong ties with both Russia and Ukraine, did not immediately respond to this request.

But Cavusoglu said Turkey cannot prevent Russian and Ukrainian ships accessing the Black Sea due to Article 19 of the convention that allows littoral states' vessels to return to their home base.

He said whether the ship is registered to the home base was clear and warned against any abuse.

"It should not be involved in the war after declaring it will return to the base and passing through the Bosphorus," the minister said.

Turkey has strong relations with both Russia and Ukraine and has offered to host talks between their leaders.

"We have repeatedly called for a diplomatic solution, but the war is a reality right now," President Recep Tayyip Erdogan's top press aide Fahrettin Altun tweeted on Sunday.