Why is Ukraine crisis threatening to upset delicate Black Sea equilibrium?

Ukrainian presidential advisor has said the country had halted an advance on the Black Sea port city of Mykolayiv after local authorities said Russian troops had entered.

“All enemy’s attempts to move directly towards Mykolayiv were repulsed,“ Oleksiy Arestovych, adviser to President Volodymyr Zelenskiy said.

If captured, the city of 500,000 people in southern Ukraine, where Russian forces have made the most progress so far, would be the biggest yet to fall.

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Ukraine's military says it is fighting "fierce battles" with Russian forces on the edge of the southern city of Mykolaiv -- which controls the road to the key Black Sea city of Odessa in the west.

Dozens of civilians are being killed in the battle for Chernihiv in the north, with those who remain living in craters or among the ruins.

The port of Mariupol has endured heavy bombardment, a sign of its strategic value to Moscow due to its position between Russian-backed separatist-held eastern Ukraine and the Black Sea Crimean peninsula, which Moscow seized from Kyiv in 2014.

Russia held nuclear drills Saturday as well as the conventional exercises in Belarus, and has ongoing naval drills off the coast in the Black Sea.

Also see |What does aftermath of Russian invasion look like for Ukraine?

In response to Russia's invasion, Turkey last month closed its Bosphorus and Dardanelles straits linking the Mediterranean and Black Seas to warships under a 1936 pact, limiting passage of some Russian vessels. The convention stipulates however, that warships belonging to Black Sea coastal countries can return to their bases.

The Italian government had also approved deployment of 235 military forces aboard two or three naval vessels, as well as an aircraft as part of surveillance and intelligence-gathering in the eastern Mediterranean and the Black Sea.

The stakes are high: if Russia manages to conquer the entire Ukrainian coastline up to the Danube Delta,one of Europe's principal shipping lanes,that would create a direct point of contact between Moscow and NATO member Romania.

If Russian forces take the southern Ukrainian city of Odessa, they could "completely take the Ukrainian coast... and consolidate their hold on the Black Sea", said Igor Delanoe, a specialist on the Russian navy at the Franco-Russian Observatory.

"Russia would thus complete what began in 2014" when they annexed the Crimean peninsula, extending their influence over the Black Sea, according to Jean-Sylvestre Mongrenier of the Thomas More think-tank, which is based in Paris and Brussels.

(With inputs from agencies)