Russia and Ukraine have accused each other of breaking international law after a Russian border patrol seizure of three Ukrainian ships.
The naval vessels – two artillery ships and a tug boat – were bound for the Ukrainian port of Mariupol in the Sea of Azov.
This required them to pass through the Kerch Strait, a narrow strip of water being blocked by a Russian cargo ship.
Both sides are blaming each other and there has been a flurry of claims and counter-claims.
Ukraine has criticised the Russian authorities, which this year started imposing checks on ships travelling to and from Ukrainian ports in the Sea of Azov.
Under a 2003 treaty, Russia has the right to inspect any vessel sailing to or from the Sea of Azov. Ukraine has accused Russia of abusing that right.
Ukraine has also opposed a Russian-constructed bridge that crosses the Kerch strait and connects Russia and Crimea.
Neither Ukraine nor the European Union nor the United States recognise Russia’s 2014 annexation of Crimea.
Kremlin-backed forces seized control of the Crimean peninsula and the territory voted to join Russia in a referendum that Ukraine and the international community deem illegal.
Russia says it had temporarily closed the area for shipping and that the Ukrainian vessels entered its territory illegally carrying out “provocative actions”.
The Kremlin’s spokesman, Dmitry Peskov, says: “Foreign military ships entered Russia’s territorial waters without responding to any requests made by our border guards. Therefore, all actions were taken in strict compliance with the law.”
Russia also says the Ukrainian vessels had not submitted the correct transit applications to “ensure safe navigation”.
The Russian Federal Security Service (FSB) accused Ukraine of violating Article 19 and Article 21 of the UN Convention on the Law of the Sea, which “outline the right of a coastal state to provide maritime security”.
Ukraine says Russia violated international law because the Black Sea is free for all shipping.
It cites the fact Ukrainian vessels have free access to the Sea of Azov and Kerch Strait under the 2003 treaty with Russia that effectively makes these waters shared territory.
“Therefore, with its actions, the Russian Federation has confirmed that bilateral agreements on the Kerch Strait and the Sea of Azov are null and void. We understand that Russia has never had any intention to follow them,” said Ukrainian Foreign Minister Pavlo Klimkin on the 112 Ukrayina TV news channel.
Two Ukrainian ships were escorted without incident by Russian tugs under the bridge several weeks ago.
Ukraine says it also warned Russia of its plan to move the ships to Mariupol through the Kerch Strait.
It maintains that Russia rammed one of its boats in an “act of armed aggression”.
Mr Klimkin told journalists Russia’s actions constituted a violation of “the freedom of maritime traffic” and of Articles 38 and 44 of the UN Convention on the Law of the Sea, which “clearly and strongly bans the obstruction of peaceful transit across the Kerch Strait”.