Turkey has closed off the Bosphorus and Dardanelles straits to warships from any country following Russia’s invasion of Ukraine.
Turkey has warned all countries (whether bordering the Black Sea or not) not to send warships via the straits to the Black Sea
Under the 1936 Montreux Convention, Turkey has control over the Dardanelles and Bosphorus straits that connect the Mediterranean and Black seas and can limit the passage of warships during wartime or if threatened.
This allows it to curb new Russian warships going to the Black Sea. The pact grants exemption to ships returning to their home bases.