Letting two Royal Navy ships through would violate the Montreux Convention, Ankara declares.

Two British minehunter ships destined for Ukraine cannot travel through Turkish waters, Ankara stressed Tuesday, citing an international pact.

U.K. Defense Secretary Grant Shapps announced last month that Ukraine’s armed forces had struck a deal to procure the Sandown Class vessels from Britain’s Royal Navy as the war-torn country grapples with Russia’s continued blockade of the Black Sea.

But Turkey confirmed it had informed its NATO allies that the ships would not be allowed to travel through its waters.