In 1997, Ukraine and Russia concluded a Treaty of Friendship which entered into force in 1999. This document was supposed to be automatically renewed every 10 years unless it was denounced by one party or the other.
In October 2018, Ukraine decided to denounce this treaty. Then it organized the incident at Kertch. Furthermore, President Petro Poroshenko gave his administration the task of listing all the agreements concluded with Russia and identifying those that he should denounce.
One of the provisions of the Treaty of Friendship and its extension in 2003 provide that the Sea of Azov falls within the territorial waters of Russia and Ukraine. As such, warships cannot enter these waters without the authorization of both states. The UN Convention on the Law of the Sea 1982 does not apply in this case.
Once the treaty and its extension is repealed, the Azov Sea will be governed by international law, with the Ukrainian and Russian territorial waters as well as an internationalized part. As a consequence, Nato ships will no longer need Russia’s consent to enter.
This explains why Nato is involved in the preparation of the incident at Kertch.