What happened on Sunday?

Depends who you ask. Ukraine says that three of its ships were travelling from Odessa to Mariupol when they were intercepted by the Russian coast guard. The Russian boats fired on the Ukrainians and also rammed one of their tugboats. Russia says three Ukrainian sailors were wounded, Ukraine says the number was six. Russia also scrambled jets and helicopters, and even blocked the Kerch strait with a barge, closing access to the Sea of Azov.

Russia doesn’t deny any of this happened, but claims the Ukrainian ships violated territorial waters. The Ukrainian ships and sailors are now being held by Russia in Crimea.

How did Ukraine react?

Angrily. Ukraine called the attack on the ships an “act of aggression” and parliament voted to declare martial law in border territories, a measure it resisted even at the worst moments of the conflict in south-east Ukraine.

Where is the Kerch strait and why is it significant?

The Kerch strait is the waterway between mainland Russia and Crimea, the peninsula Russia annexed from Ukraine in 2014. It serves as the gateway into the Sea of Azov, which borders Russia and Ukraine. A bilateral treaty gives both countries the right to patrol the waters. It is also the site of a new, 19-km (12-mile) bridge built by Russia that cost an estimated $4bn (£3.1bn) and has been touted as a prestige project. Russia has significantly built up its military presence in the region since 2014.

What is the state of Russia-Ukraine relations?

They have never been worse. After a revolution in Ukraine toppled the previous president, Russia annexed the peninsula of Crimea and backed a separatist revolt in south-east Ukraine. Ukraine has responded by cutting ties with Russia and asking for support from the west. More then 10,000 people have been killed in south-east Ukraine since 2014 and the conflict is ongoing.

How has the international community reacted?

A number of western countries have said they are “deeply concerned” but it is not clear that will be backed up by actions, including new sanctions. Donald Trump gave a subdued response to the clash, saying “we do not like what’s happening either way”. He is set to meet Vladimir Putin during the G20 summit in Argentina later this week. US secretary of state Mike Pompeo later issued a statement saying that the US “condemns this aggressive Russian action”.

The UN secretary general, António Guterres, has urged both countries to use “maximum restraint” and avoid further escalation.

What is Putin thinking?

Two Ukrainian gunboats and a tug hardly presented a threat to the Russian navy. The Kremlin is throwing its weight around to show that it will control the waters near Crimea despite treaties or international condemnation to the contrary. There is also speculation that a military crisis could boost domestic support for Putin, whose favourable ratings are at their lowest in years because of painful pension reforms. In fact, the last time the Russian president’s favourables were in the mid-60s (low for Russia), he annexed Crimea and saw his ratings rise above 85% (at least according to Russian pollsters).

What might happen next?

Russia has indicated it is going to charge the sailors, which will provoke further anger from Kiev. Ukraine says the sailors should be treated as prisoners of war, which would preclude a criminal trial. Most likely, Russia will seek to use them as a bargaining chip in further trades with Ukraine, or use them as a way to put pressure on the government. For now, the Kerch strait is open, but Russia has shown it can and will close it to Ukrainian ships when it serves Moscow’s interests.