Fears that Moscow is preparing for an invasion by massing 100,000 troops on its border with Ukraine -- itself not a NATO member -- have made bolstering the Atlantic alliance's presence on the strategically vital Black Sea more pressing.

In response to an attack on Ukraine, the United States would "increase troop presence in Poland, in Romania, etcetera... because we have a sacred obligation in Article 5 (of the NATO treaty) to defend those countries. They are part of NATO," Biden said Wednesday.

The same day, Macron said France was "available" to "commit itself to new missions... especially in Romania, if they are decided" by the alliance.

NATO already has "enhanced forward presence" (EFP) commitments in Baltic nations Estonia, Latvia and Lithuania as well as Poland to defend them against any Russian attack.

Britain, Canada, Germany and the United States each lead a deployment in one of the four countries.

"Until now, there has been a difference between the northeast and southeast of Europe" regarding troop deployments, a European diplomatic source told AFP.

If a Romania mission goes ahead, "there would be the same level of commitment" as in the Baltic, the source added.

- Russian demands -

But Russia made clear Friday that its security demands of the US-led alliance include withdrawal of troops from the Black Sea region.

Russia wants the "withdrawal of foreign forces, hardware and arms" from countries that were not NATO members before 1997, Moscow's foreign ministry said.

"These include Bulgaria and Romania," it added, singling out the two former Warsaw Pact allies, who joined NATO in 2004.

In the run-up to talks between Moscow's top diplomat Sergei Lavrov and US counterpart Antony Blinken in Geneva Friday, the United States declared Russia's claims on its former sphere of influence a "non-starter".

For Russia, the Black Sea is crucial for the access it offers to crisis flashpoints in the Middle East and Africa.

The region is also the backyard of Turkey, a NATO member close to Moscow whose relations with France have at times been tense.

In 2019, Turkey's interventions in Syria -- uncoordinated with its treaty allies -- prompted Macron to say NATO was suffering "brain death".

Macron's willingness to send troops to Romania appears to signal greater readiness to commit to the alliance in France, which is a longstanding but at times aloof member.

- Closer to Bucharest -

Any decision on a NATO deployment will likely not come before next month at the earliest, a fact recalled this week by Bulgaria's President Roumen Radev, seen as close to Moscow.

"There is no military solution to these crises, they require dialogue and diplomatic effort at de-escalation," he said.

But in Romania, President Klaus Iohannis tweeted Thursday that "I warmly welcome President Emmanuel Macron's announcement on France's readiness to participate in NATO's forward military presence in Romania."

"The Romania-France strategic partnership will thus be reinforced on the Eastern flank, in the Black Sea region," he added.

For Bruno Tertrais of the Foundation for Strategic Research think-tank in Paris, this is "a significant re-orientation" by France.

"After making a significant, long-term investment on the Baltic, France wants a presence in the Black Sea region. And to strengthen its ties with a long friendly country that we've neglected too much," he added.

George Scutaru, head of Romania's New Strategy Center think-tank, agreed that it would be "very significant" if France takes the lead.

As France's eyes turn to NATO's eastern flank, Defence Minister Florence Parly this month met counterparts from the eastern EU "B9" group including Bulgaria, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania and Slovakia.

"It was symbolically important that France should organise this for the first time, usually it's been the US," said one diplomat from the group.

"Everyone welcomed the fact that France is taking an interest on the eastern flank."

France's interests closer to home may benefit too, with Romania set to deploy special forces to aid the French-led mission against jihadism in the sub-Saharan Sahel region.

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