Russian border guards opened fire on three Ukrainian naval vessels in the Kerch Strait, near the Russia-occupied Crimean peninsula on Sunday night, raising the prospect of a full-scale military confrontation between the two neighbours.
The Federal Security Service, known as the FSB, confirmed on Monday that it fired on Ukrainian vessels to stop them. Russia claims that passing of Ukrainian military boats in the Kerch Strait, between Russia and annexed Crimea, is a provocation against its maritime territory.
In response, Ukraine imposed martial law (which will go into effect on Wednesday) and demonstrations were held outside the Russian Embassy in Kiev to protest Moscow's actions on Monday.
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Is it merely a one-night stand-off?
This potentially dangerous escalation in the crisis between the two countries prompted Kiev to put its forces on full combat alert and look into enforcing martial law.
The incident comes on the back of a four-and-a-half year long proxy conflict in eastern Ukraine and recent rising tensions over ships navigating the Sea of Azov. a body of water north of the Black Sea shared by Ukraine and Russia.
Tensions have been rising in the Kerch Strait for a few weeks now.
Shortly after Ukraine detained a fishing vessel travelling from Crimea in March, Russia increased its military presence in the area and started inspecting all vessels travelling to or from Ukrainian ports, causing days-long delays and disrupting trade.
Ukraine has protested the checks, calling them an "economic blockade."
Poroshenko's martial law
Ukrainian President Petro Poroshenko signed a decree on Monday to introduce martial law for 60 days, a statement on his website said, though it needs parliamentary approval to come into force. He later halved the duration in an apparent concession to opponents and the measure was passed by parliament.
Eleven measures outlined in the decree provide for mobilising reserve forces, organising the air defence of important state facilities and taking urgent steps to bring in enhanced cybersecurity measures and ensure public order.
Ukrainian legislation on martial law also allows restrictions on movement and media, though the decree made no specific references to these.
What makes Azov a prickly issue to navigate?
The Sea of Azov is located between Russia and southeast Ukraine.
A bilateral treaty gives both Russia and Ukraine the right to use the Sea of Azov, which lies between them and is linked by the narrow Kerch Strait to the Black Sea.
However, in order to access to the sea, ships should pass the Kerch Strait, where both sides are controlled by Russia.
The strait is spanned by the $3.6 billion Kerch Bridge, which Russia built to connect annexed Crimea to Russia. The bridge is too low for certain vessels to pass through, further hampering trade, something that has incensed Kiev.
Ukraine has accused Russia of persistently detaining ships sailing to and from its ports on the Azov Sea, especially Mariupol and Berdyansk, with a view to disrupting trade.
Mariupol is also the closest government-controlled city to Donetsk and Luhansk, the breakaway regions of eastern Ukraine controlled by Russia-backed separatists.
Poroshenko, who spoke of the importance of the port of Mariupol to the Washington Post, said blocking Ukrainian vessels carrying iron and steel even for one day costs the economy thousands of dollars. Iron and steel products from that port account for “25 percent of Ukraine’s export revenue,” Poroshenko said. “Then the Russians are attacking Ukrainian fishermen in Ukrainian waters all the time — the Russians arrest them, stop them and endanger them.” he told the Post.
Russia in turn accuses Ukraine of harassing Russian ships, and says its own checks on Ukrainian vessels are lawful and necessary to ensure the security of the area.
Squabbles over control of the Azov Sea and the Kerch Strait are nothing new. Tensions flared in 2003 during Vladimir Putin’s first term as Russian president.
These were calmed with a 2003 bilateral treaty stipulating that both countries could use the Kerch Strait and Azov Sea freely for commercial shipping and must notify each other while sending military vessels.
Going back to the annexation of Crimea
Russia seized the majority Russian-speaking Crimea from Ukraine on March 16, 2014 after an uprising toppled Ukraine’s pro-Russian president and a pro-Western government took power in Kiev.
Ukrainian troops largely did not put up resistance and retreated. Russian officers forced local lawmakers to pass a motion calling for a referendum to secede and join Russia. A majority of votes cast on March 16, 2014, supported the motion. Two days later, Moscow signed a declaration with self-proclaimed Crimean officials, sealing the annexation.
Neither the vote nor the annexation is recognised by the United Nations.
At least 13,000 people have died since the conflict began, and 1.7 million have been internally displaced.
Ukraine and NATO accuse the Kremlin of supporting the separatist rebels with troops and weapons.
Why is Russia blaming Poroshenko?
Dmitry Kiselyov, a commentator on Russian state-controlled TV, told viewers of his Sunday evening news programme that Poroshenko, encouraged by the US, is looking to pick a fight with Russia in the Black Sea.
The talk show host also alleged the US talked Poroshenko into staging a provocation against Russia as a means to disrupt an upcoming meeting between Presidents Vladimir Putin and Donald Trump at this week's Group of 20 summit in Argentina.
"What is happening now at the [Kerch] bridge threatens to turn into a very unpleasant story," Kiselyov said.
Some Russian politicians have accused Poroshenko of deliberately instigating the stand-off to boost his flagging popularity ahead of elections in March.
Ukrainian opposition politicians have speculated that he is using the introduction of martial law as an excuse to postpone the elections.
Poroshenko's reduction of the term means that Ukrainian authorities can call the presidential election at the end of December, giving enough time for it to take place at the end of March as expected.
What happens next?
Though Poroshenko is likely to get parliament’s backing for martial law — a televised session of a parliamentary committee unanimously supported the decree — military escalation by Ukraine risks inviting a forceful reaction from Russia, whose Black Sea fleet is stationed in Crimea and outguns the Ukrainian Navy.
At Kiev’s urging, its Western allies could push for more sanctions on Russia, a prospect which pushed the rouble lower on Monday.
The United Nations Security Council will discuss the crisis on Monday at the request of Russia and Ukraine.
NATO and the European Union urged restraint on all sides and urged Russia to restore full access to the Sea of Azov for commercial vessels.
NATO said it will hold an emergency meeting with Ukrainian officials at alliance headquarters in Brussels on Monday.
German Foreign Minister Heiko Maas called the Russian blockade of the Sea of Azov "unacceptable" and urged an easing of tensions with Ukraine following a weekend flare-up.
Poland, Denmark and Canada condemned what they called Russian aggression.