Ukraine’s parliament is to decide whether to bring in martial law, after Sunday’s capture of three of its naval vessels and 23 crew members by Russia.
The three ships were sailing off the coast of Crimea, which was annexed by Russia in 2014, when they were seized.
Russia opened fire, before its special forces stormed the vessels. Between three and six Ukrainians were injured.
Ukraine said it was a Russian “act of aggression”. Moscow said the ships had illegally entered its waters.
The incident marks a major escalation of tension between the two countries.
This is the first time the two militaries have come into open conflict in recent years, although Ukrainian forces have been fighting Russian-backed separatists and Russia volunteers in the east of the country.
Kiev also says Russian regular troops have fired on Ukrainian positions a number of times in the past.
A number of Western countries condemned Russia’s actions, and the UN Security Council is due to hold an emergency meeting.
Sunday’s naval clash – what happened?
The Ukrainian navy said its vessels – two gunboats and a tug – were hit and disabled on Sunday evening as they tried to leave the area near a Russian-built bridge over the Kerch Strait – the only access to the Sea of Azov.
It said all the crew members – including six injured sailors – were captured by Russian special forces.
Ukrainian military chief Viktor Muzhenko said the Russian coastguard had “fired to kill” during the confrontation.
Russia’s FSB security agency said its coastguard ships chased the Ukrainian ships and opened fire to force them to stop.
It also said three Ukrainian crew members were injured and were now being treated in a hospital in Crimea.
Before the clash, Russia scrambled fighter jets and helicopters, and also blocked the bridge with a tanker.
What led to this?
On Sunday morning, Ukraine’s vessels tried to sail from the Black Sea port of Odessa to Mariupol in the Sea of Azov.
Ukraine said Russia then tried to intercept the boats, ramming the tug.
Russia accused the Ukrainian ships of illegally entering its waters, after the FSB had temporarily closed an area of water for shipping.
Kiev called it a flagrant violation of international law, because the Black Sea is free for shipping, and annexed Crimea belongs to Ukraine.
Ukraine also cited a 2003 Russia-Ukraine treaty on unimpeded access to the Kerch Strait and Sea of Azov.
It said it had informed the Russians in advance of its plan to move its ships through the sea to Mariupol – a claim denied by Russia.
In recent weeks, two Ukrainian vessels passed through the Kerch Straight without incident.
- The crisis as it happened
- Ukraine names Russia as ‘aggressor’ state
‘A difficult balance’
By Jonathan Marcus, defence and diplomatic correspondent
The incident in the Black Sea is a powerful reminder that the tensions between Russia and Ukraine are not part of a frozen conflict: they can flare up with very little warning.
Nato and Ukraine’s allies in the West have strongly backed President Petro Poroshenko. But what can they do to influence Russian behaviour?
There will be talk of more economic sanctions. But Russia is already heavily sanctioned and this has not encouraged it to rethink its annexation of Crimea. There will be calls for additional support for the Ukrainians; Nato countries provide training for Kiev’s military – they could presumably do more.
And the Trump administration, even before this episode, was already considering calls to sell additional weaponry to Ukraine in addition to the Javelin anti-tank missiles already supplied.
But there is a difficult balance to be struck between support for Ukraine’s territorial integrity on the one hand and avoiding anything that might tip the conflict into full-scale war.
Why is this happening now?
There have been growing tensions between the two sides over access to the Black Sea and the Sea of Azov.
In recent months, Russia has begun inspecting all vessels sailing to or from Ukrainian ports.
The inspections began soon after Ukraine detained a fishing vessel from Crimea in March. Moscow says the checks are necessary for security reasons, pointing to a potential threat to the Kerch bridge from Ukrainian radicals.
Ukraine has accused Russia of trying to occupy the Azov sea and damage Ukraine’s economy by hindering access to two important ports, Berdyansk and Mariupol.
President Poroshenko told the Washington Post in September that iron and steel products from Mariupol made up 25% of Ukraine’s export revenue.
How has Ukraine reacted?
The stand-off has been met with anger in Ukraine.
Late on Sunday, crowds gathered outside the Russian embassy in Kiev, some throwing flares. At least one car belonging to the embassy was set alight.
President Poroshenko described the Russian actions as “unprovoked and crazy”, and said he would ask parliament on Monday to introduce martial law. He has now signed the decree requesting parliament to do so.
He stressed that this did not mean a “declaration of war… Ukraine does not plan to fight anyone”.
The Ukrainian defence ministry announced that orders had been given to put the military on full combat alert.
What could martial law in Ukraine involve?
Martial law could give the government the power to restrict public demonstrations, regulate the media, suspend elections, and oblige citizens to carry out “socially necessary” tasks such as working at a defence facility, local media report.
Ukraine’s parliament is currently discussing how to proceed with the issue. A vote is expected later on Monday.
If approved, it would be the first time Ukraine has enacted martial law since the beginning of the Ukraine-Russia conflict in 2014.
However, politicians are split on the issue, with some expressing concern that it could lead to the 2019 presidential and parliamentary elections being cancelled.
Some say President Poroshenko could be a main beneficiary, as his ratings have plummeted in recent months.
Why are relations so bad between Russia and Ukraine?
Ukraine gained independence after the collapse of the Soviet Union in 1991.
However, Russia considers a Western-leaning Ukraine a threat to its interests.
In 2014, Ukraine’s pro-Russian leader was overthrown, after large-scale protests against the government’s decision to abandon plans to sign an association agreement with the EU.
Russia then annexed Crimea, while Russia-backed separatists in the eastern Donetsk and Luhansk regions moved against the Ukrainian state.
Ukraine and the West accuse Russia of sending its troops to the region and arming the separatists.
Moscow denies this but says that Russian volunteers are helping the rebels.
More than 10,000 people have been killed in the conflict in the east.