Russia’s seizure of three Ukrainian warships in the Black Sea is one of the most ominous incidents in Moscow’s nearly five-year campaign of military, political and economic pressure on Kiev. Ukraine says six of its seamen were injured when Russian coast guards opened fire on the ships on Sunday. This is the first time Russia has admitted its own forces, rather than “volunteers” in unmarked uniforms or the Kremlin’s surrogates in east Ukraine, engaged directly with the Ukrainian military. The risk of escalation — with at least the potential to suck in Nato forces — is dangerously high.

Ukraine’s decision to sail a military tug and two gunboats through the Kerch Strait between Russian-occupied Crimea and Russian territory may have been a deliberate attempt to capture attention, one week before Russia’s Vladimir Putin is expected to meet US president Donald Trump at the G20 summit. But a 2003 treaty designates the strait and the Sea of Azov, which it links to the Black Sea, as shared territorial waters between Ukraine and Russia. Kiev says it went beyond its legal obligations in notifying Moscow of its intentions in advance.

The move followed months of what amounts to a creeping annexation by Moscow of the Azov Sea, since Russia opened its multibillion-dollar Kerch Strait bridge linking Crimea to Russian territory in May. The bridge’s central span is too low to allow passage of many ships that used to dock at two Ukrainian Azov Sea ports that are vital export terminals for steel and grain. Under the pretext of protecting the bridge, Russia has been stopping Ukrainian commercial vessels for lengthy checks, and building up its naval presence in the Azov Sea.