Russian border guards intercepted and seized three Ukrainian navy boats off Crimea on 25 November, in a major escalation of the Russia-Ukraine conflict.
Why did this happen?
Two Ukrainian gunboats and a tug were sailing towards the Kerch Strait, the only route for ships to enter the Sea of Azov from the Black Sea.
Russia's FSB border guard force says the flotilla violated Russian territorial waters.
But coordinates released later by the FSB and Ukraine confirm that the Russian attack happened in international waters near the strait.
Ukraine calls it Russian aggression, because the Black Sea is free for shipping and annexed Crimea belongs to Ukraine.
A 2003 Russia-Ukraine treaty stipulates unimpeded access to the Kerch Strait and Sea of Azov.
How serious is this?
It is the most dangerous clash at sea off Crimea since Russia annexed the Ukrainian peninsula in 2014.
Ukraine's President Petro Poroshenko urged Nato to send ships to the Sea of Azov, warning of a threat of Russian invasion.
Nato shows no sign of doing so - Ukraine is not a member - but supports Ukraine. Western leaders condemned Russia's actions.
Mr Poroshenko has put Ukraine's border regions under martial law until 26 December and barred Russian men aged 16-60 from entering, except for "humanitarian cases".
Russian President Vladimir Putin accused him of staging a "provocation" to boost his poll ratings.
Russia is holding the three boats in Kerch. One was rammed by an FSB vessel in the clash.
The Russian forces opened fire and several Ukrainian sailors were injured. All 24 are in Russian detention.
How does it affect the conflict?
It could flare up again. The pro-Russian separatists in eastern Ukraine have Russian heavy weapons; Ukraine has Western support. They have been fighting since April 2014.
Russia's new bridge over the Kerch Strait, opened in May, tightened its grip on Crimea.
Russia regularly inspects Ukrainian ships in the Sea of Azov, an area vital to the Ukrainian economy.