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".
In saying that the Ukrainian ships were seized in its territorial waters, Russia is arguing that the vessels were in Russian "territorial sea", which extends up to 12 nautical miles from a country's coastline.
Ukraine considers Crimea as its own and could therefore argue that travelling in waters off the coast of Crimea is effectively moving through Ukrainian territorial waters.
Under the 2003 agreement between Russia and Ukraine, regardless of the status of Crimea itself, Ukraine argues, it has "freedom of navigation" in the Sea of Azov as well as access to it through the Kerch Strait.
The law of the sea
The UN Convention on the Law of the Sea (UNCLOS) sets out various scenarios that give a state freedom of passage, irrespective of a state's territorial waters.
All ships, including foreign warships, enjoy the right of "innocent passage" within another state's territorial sea under international law.
Russia has disputed whether the passage was innocent. The UN law states that a passage is innocent "so long as it is not prejudicial to the peace, good order or security of the coastal state". That includes threat or use of force, exercise or practice with weapons or any act of propaganda affecting the security of the state.
Russia would need to prove that the passage of the Ukrainian vessels was not innocent and that Ukraine had showed "some form of hostile intent", says Mr Muller, to act against them.
According to the Russian FSB's account, Ukrainian vessels entered "combat readiness" in contravention of the innocent passage rules.
A country does not need to ask for permission before exercising that right but can be asked to follow certain rules once doing so.
This may include measures to protect security interests, says Dr Wim Muller, an international law expert at Chatham House.
Russia has pointed to a section of this UN convention that requires a warship to leave its territorial waters if it fails to comply with the laws of that country.
Under international law, a country would have the right to seize another warship only if the warship was acting in a hostile manner, says Valentin Schatz, a research associate in public international law at Germany's University of Hamburg.
Ukraine has also highlighted provisions (Article 38 and Article 44) of the convention, which require all ships to be given the freedom to travel through a strait from one part of the high seas to another - known as transit passage.
There are also rules within the UN convention that "ensure that ports which can only be reached by a single route through the strait, as is true of all ports in the Sea of Azov, always remain accessible", says Andrew Serdy, director of the Institute of Maritime Law at Southampton University.