Turkey on Friday refused a request by Ukraine to deny Russian warships passage through the Bosporus and Dardanelles Straits to the Black Sea, citing a clause in the Montreux Convention that allows ships to return to their home base.

Turkish foreign minister Mevlüt Çavuşoğlu announced Turkey’s decision, telling Hürriyet Daily News that the country could not fulfil Ukraine’s request.

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“In the case of a war where Turkey is not a party, there are measures that can be taken regarding the countries that are parties in the war,” he said.

“If there is a demand for the ships of the warring countries to return to their bases, then it must be allowed,” Çavuşoğlu added, referring to Russian warships.

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“Russia had its own clause put in [to the Montreux Convention]. Articles 19, 20 and 21 stipulate that if the ship of a country that is on the Black Sea wishes to pass with a request to return to its base, this is allowed,” the Turkish foreign minister explained.

For those unaware, the Montreux Convention was signed on July 20, 1936 at the Montreux Palace in Switzerland and gave Turkey control over the Bosporus and Dardanelles Straits with regard to the transit of naval ships. The agreement came into effect a few months later on November 9, 1936.

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The convention, which remains in force to this day, guarantees the free passage of civilian ships during peace time, but restricts the passage of naval ships that do not belong to Black Sea states.

Since the Black Sea states include Russia, Ukraine, Turkey, Bulgaria, Georgia and Romania, it is understood that Turkey cannot block the passage of Russian warships.