The clash in the Sea of Azov is the most dangerous head-to-head off the coast of Crimea since Russia annexed the Ukrainian peninsula in 2014. Ukraine has imposed martial law along parts of its border, which Russia said was an extreme measure. The UN Security Council is now planning to hold a special meeting, requested by Russia and Ukraine, as fears of a new bloody chapter in the region grow.
Why are relations so bad?
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.
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Russia then annexed Crimea, while Russia-backed separatists moved against the Ukrainian state in the east.
More than 10,000 people have been killed in the conflict in the east.
What prompted this most recent development?
There have been growing tensions between the two sides over navigation in the Sea of Azov, which is shared by Russia and Ukraine.
Russia has recently begun inspecting all vessels sailing to or from Ukrainian ports in the Sea of Azov, which began after Ukraine detained a fishing vessel from Crimea in March.
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Moscow also says the checks are necessary for security reasons.
Ukraine has accused Russia of trying to occupy the Sea of Azov and damage Ukraine’s economy by hindering access its ports.
What happened on Sunday?
On Sunday, November 25, two Ukrainian gunboats and a tug were sailing into the Kerch Strait of the Crimean peninsula, the only route for ships to enter the Sea of Azov from the Black Sea.
The vessels were stopped from entering the Kerch Strait and confronted by Russian FSB border guards.
After a lengthy standoff, during which the Ukrainian tug was rammed, the ships began turning back towards Odessa, the Ukrainian government says.
The Russians then opened fire, wounding at least three sailors, and seized the Ukrainian flotilla.
The crew members were then captured – 24 in total – and have now been given two months in pre-trial detention.
Mr Poroshenko told Sky News he has evidence that Russian military tanks are massing along his country’s border, near where Russia stores its ammunition.