evidence: Last Sunday, two small Ukrainian artillery ships and a tugboat were prevented by Russian vessels from crossing the narrow Kerch Strait and reaching the Ukrainian port of Mariupol on the Sea of Azov, which both countries share. [...] Mariupol sits on the Sea of Azov, a small, shallow body of water that is shared by Russia and Ukraine under a 2003 treaty. Its only outlet to the Black Sea is through the Kerch Strait, a 15km-wide channel that is spanned by a recently completed and highly controversial bridge linking Russia to annexed Crimea. [...] Kiev and the West are also calling on Russia to end a de facto blockade of Ukrainian ports in the Sea of Azov, which this week prevented dozens of commercial ships from entering or leaving the area through the Kerch Strait.