So a little background for those not following this situation. The Azov sea is a small sea sandwiched between Russia and Ukraine, in the north-eastern portion of the Black Sea. The only real way in and out (other then rivers) is the Kerch straight. This straight passes between Crimea, and Taman' (a piece of southern Russia). Over a number of years after the fall of the Soviet Union, agreements were signed between Russia and Ukraine about both border demarcation, and joint use of the sea lanes. The agreement included a vague clause about detaining suspicious ships for inspection (rarely used). After the annexation of Crimea, the agreement became outdated, since Russia now controlled both sides of the straight, but joint use of the sea lanes continued anyway until the current crisis. The current crisis began with the arrest of a fishing boat called Nord, out of Crimea. It was stopped by Ukrainian coast guard, and arrested for violating rules about entering and exiting the occupied territory of Crimea. In other words, the boat was arrested for being Russian, and being out of Crimea. Following this, Russia attempted to exert diplomatic pressure to secure the release of the ship and crew. Eventually the entire crew, except the captain, were released. As a symmetrical response, Russian FSB coast guard stepped up patrols of exclusive Russian fishing areas, and arrested multiple Ukrainian vessels for fishing illegally, over several months. This has dealt a major blow to Ukrainian fisheries who, over the past ~25 years have become used to being able to fish in Russian waters, despite the laws and regulations governing this. In the past enforcement was extremely lax, often bordering on non-existence, and bribes could frequently secure unfettered access. Russian coast guard also started using the clause about inspecting suspicious ships to stop nearly all ships headed to Ukrainian ports, and detain them for searches, subsequently releasing them. While this might seem harmless, this delayed shipping from several hours, to several days (the longest wait period I'm aware of was 11 days) causing losses to shipping companies, and effectively shutting down sea traffic to the major Ukrainian port of Mariupol'. As a response to this, Ukraine detained a Russian river boat, the Mekhanik Pogodin. First it was alleged that the company owning it was under sanctions, this turned out to be untrue, then it was alleged that the ship had violated Ukrainian environment regulations, and "environmental inspectors" attempted to force their way aboard the ship. When they were denied access, Ukraine arrested the ship for 3 years, and threatened to confiscate and sell it. Russia also rebased many additional ships for both the coast guard and the VMF, to strengthen their presence in the Azov. To respond to the situation, Ukraine decided to rebase a number of warships (or "warships" if you like, given their nature) to the Azov sea. Several Gyurza-M boats were brought overland by trailers, and a Ukrainian command ship (aptly named the Donbass) and the tugboat named Korets approached the Azov sea from the Black Sea. They requested permission to pass, accepted a Russian navigator on board, and passed into the Black Sea, following all the same rules and regulations that normally governed the passage of ships in Russian territorial waters. Despite this rather routine action, Ukrainian president Poroshenko congratulated his ships and crews, and Ukrainian press was full of claims that the Ukrainian Navy "surprised" Russia by passing through the Kerch straight. In Ukrainian sources this was touted as a victory of some sort. Finally just yesterday, a far more dangerous incident occurred. Two Ukrainian Gyurza-M boats, and a tugboat, attempted to enter the Kerch straight. They requested permission to pass, but refused to accept a Russian navigator, or take their place in the regular order of sea traffic passing through the straight. When Ukrainian vessels entered Russian territorial waters (off Crimea) they were hailed repeatedly by FSB coast guard but refused to comply. Consequently a Russian coast guard ship, the Don, rammed the tug boat, causing significant damage to the engine and rear of the vessel. Following this, Ukrainian ships left Russian territorial waters, and are currently standing by south of the Kerch straight. In the mean time Russia has closed the Kerch straight entirely, going so far as to physically block the straight with a cargo ship. They are citing safety reasons, due to potential for Ukrainian provocations. Two Su-25 dive bombers, and two Ka-52 attack helicopters have been on station almost non-stop since the incident. Two other Ukrainian gun boats are approaching the straight from the north, out of Berdyansk, but so far have not been involved in the current incident. A Russian VMF mine trawler was also spotted in the area. There have been reports that the straight was re-opened and Ukrainian ships allowed to pass through, but they appear to be false. The straight was shortly opened for the passage of Russian vessels, then closed again. Currently negotiations are continuing about the passage of Ukrainian ships. Presumably Russia's main requirement is that they accept a Russian navigator for passing the straight, and do so in the same manner as regular civilian vessels using those sea lanes. Personal comment: The timing of this latest incident is very telling, since Ukraine is introducing a UN resolution on the same day, condemning Russian militarization of the Azov sea.