Turkey has urged Russia and Ukraine to respect its decision to close the Black Sea entrance to warships.
Ankara sought to avoid an escalation in Russia's invasion of Ukraine by shutting down the straits that lead to the Black Sea.
Warships would be blocked from accessing the Bosphorus and Dardanelles straits, whether they come from countries "bordering the Black Sea or not", Foreign Minister Mevlut Cavusoglu announced.
"We are applying to the letter the provisions of the Montreux Convention," he told reporters on Monday.
The 1936 international agreement allows Ankara to manage war traffic entering the two sea lanes during times of conflict.
If Turkey is involved in the war, the treaty allows Ankara to take any decision it deems appropriate.
Ships that are registered to bases in the Black Sea will still be allowed to return to their ports, including in NATO countries Bulgaria and Romania.
The decision means Russia will no longer be able to send naval units from other areas to the Black Sea. Cavusoglu announced that the Montreux Convention would be "strictly and fully respected".
"We have decided to use the Montreux Convention to prevent the escalation of the crisis," Turkish President Recep Tayyip Erdogan said after a cabinet meeting.
Erdogan has labelled the Russian invasion of Ukraine "unacceptable" but initially rejected a request by Kyiv to prevent Russian vessels from accessing the Black Sea.
Amid Russia's assault on Ukraine, Turkey is trying to balance its support for Kyiv with its fragile economic ties to Moscow.
Late on Monday, President Tayyip Erdogan spoke to Belarusian leader Alexander Lukashenko about hosting possible ceasefire talks between Russia and Ukraine.