As we said multiple times before, there are no problems with the passage of Ukrainian warships from the Black Sea to Ukrainian ports in the Sea of Azov. The only condition is compliance with safety standards for navigation and access to the Kerch Strait. […] So was it until November of the last year , but then […] in November  the [Ukrainian] warships secretly tried to sneak through the Kerch Strait by entering [Russian] territorial waters, the part that was Russia’s territorial waters before the referendum in Crimea.
In November and December 2018, Ukrainian civilian ships and warships wanting to enter the Azov Sea had enormous problems. Russia was deliberately blocking the Kerch Strait in order to prevent the passage of a naval group of the Ukrainian Navy.
According to UNIAN, Russia blocked the passage of over 100 ships in December 2018 - it is therefore wrong to state, that there is no problem with the passage of Ukrainian ships to the Azov Sea. Russia used a cargo ship under the Kerch Bridge to block passage under the bridge’s arch.
On November 25th 2018, three Ukrainian war ships were moving “in accordance with the provisions of all effective multilateral and bilateral international treaties and navigation rules”. The Ukrainian war ships were seized and their crews were detained, despite the fact that the UN Convention on the Law of the Sea (UNCLOS) states that “all ships, including foreign warships, enjoy the right of “innocent passage” within another state’s territorial sea under international law.”
On 25 November, border patrol boats belonging to Russia’s FSB security service seized the two small Ukrainian armoured artillery vessels and their whole crews after shooting at them, wounding several Ukrainian servicemen.
A bilateral treaty between Russia and Ukraine, signed in 2003 and ratified by Russia in 2004, governs the use of the Kerch strait and the Sea of Azov, which in the treaty is considered to be the “internal waters” of both Russia and Ukraine.