Foremost, it is about international legal obstacles to NATO naval presence caused by the status of the Sea of Azov and the Black Sea.
1. Status of the Sea of Azov. The Treaty on Cooperation in the Use the Sea of Azov and the Kerch Strait of 2003 defines the passage of combat and commercial vessels into these water areas. This treaty has many disadvantages for Ukraine, in particular, provisions which specify that the Sea of Azov is internal waters of the two countries and permit the entrance of combat vessels of third countries only by mutual consent of Ukraine and Russia. As a result, Russia is unlikely to agree on the presence of any NATO warship in the Sea of Azov.
2. Status of the Black Sea. Before warships of Ukraine’s allies may theoretically approach the Sea of Azov, they have to enter the Black Sea which also has a specific legal status. The Montreux Convention Regarding the Regime of the Black Sea Straits of 1936 imposes restrictions on tonnage (an aggregate tonnage no more than 30,000 tons) and stay (no longer than 21 days) of all non-Black Sea warships in the Black Sea. Thus, Ukraine may theoretically rely only on an increased number of Turkish, Bulgarian or Romanian combat vessels near the Ukrainian Black Sea coast, ruling out the naval presence of the U.S., the UK or any other NATO country.
Therefore, the conflict in the Sea of Azov and the Kerch Strait demonstrated that Ukraine has not only a limited military and economic potential to protect its maritime interests but the country has also to take into account international legal restrictions when it comes to foreign naval assistance.