With 240 crew and a helicopter on board, the Regele is due to depart next week for manoeuvres off Romania's coast and in international waters.

"We are going to try not to contribute to an escalation" in tensions with Russia, frigate captain George-Victor Durea said, standing on the docks of the military port, not far from huge cranes moving cargo for commercial shipping.

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.

- High stakes -

According to MarineTraffic data, practically no vessels with their automatic identification system (AIS) turned on are currently moving around in the Black Sea off Ukraine, north of a line between Romanian port town Sulina and Yevpatoria city in Crimea.

"So far the weather has not been very favourable, but we feel that the Russians are preparing to move from the Black Sea, where they have concentrated 40 warships, equipped with Kalibr missiles," a European military source said.

At the Danube Delta, where the river flows into the Black Sea, Romania and Ukraine share 110 kilometres (68 miles) of border.

This is a "very important" part of their longer frontier, and Romania has a "strong riverine flotilla monitoring the situation", according to Romanian navy spokesman Colonel Corneliu Pavel.

Controlling the coastline would also allow Russia to link up with its troops stationed in Transnistria, a separatist and Moscow-backed territory in Moldova, putting pressure on the small country wedged between Romania and Ukraine.

"This will be the big story coming up in the next weeks, the sovereignty of Moldova -- it's a big question," said University of Glasgow researcher Nicholas Myers.

- Possible sea blockade -

Within the Black Sea itself, Russia last week seized Snake Island, an uninhabited but strategic rocky outcrop just 45 kilometres from Romania's coast where Bucharest has offshore gas reserves.

Bucharest and Kyiv both claimed the island, before the International Court of Justice awarded it to Ukraine in 2009.

"It is certain that Russia will never leave the island, and that it can use it in the future to harass ships leaving the delta," said George Scutaru of Romanian think-tank New Strategy Center.

According to Pavel, Russia has installed some radar on the island.

Captain Durea said it was a "possibility" that Russia could install weapons to deter any ships from approaching, effectively erecting a sea blockade of Ukraine.

On the Black Sea's southern shores, NATO member Turkey finds itself walking a diplomatic tightrope.

Ukraine is Turkey's closest post-Soviet ally, but Ankara has also worked hard to forge ties with Moscow.

Bound to block access to battleships in wartime under the 1936 Montreux Convention, Turkey has barred warships from using the key Bosphorus and Dardanelles waterways that Russia needs to access the Black Sea from the Mediterranean.

For NATO, the holding of an exercise scheduled for April has been thrown into question as some vessels cannot pass at the moment.