While Macron did not provide any details, the French foreign ministry said the idea did not foresee French troops directly fighting against Russian forces in Ukraine.

Speaking to reporters, Lithuanian Defence Minister Arvydas Anusauskas said on Tuesday: "We are talking about training missions, training components. Training."

Former senior NATO official Camille Grand said Macron's statements were a "major political signal."

"The message is threefold: to the Ukrainians, we are saying that we are prepared to take risks alongside them. To Russia, this war is very important to us," Grand told AFP.

"To the public, the stakes are so high that we can't rule out this possibility."

- 'Good sign' -

While Kyiv has not publicly asked the West to send in troops, Ukrainian President Volodymyr Zelensky's aide Mykhailo Podolyak told AFP Macron's remarks were a "good sign", showing "a deep understanding of the risks posed to Europe."

Monday's remarks appeared to represent a turnround for Macron, who has for many years sought to position himself as a top mediator between Russia and Ukraine.

Less than five years ago he hosted Russian leader Vladimir Putin at his Mediterranean residence and flew into Moscow in February 2022 in a last-minute bid to halt the dispatch of Russian troops across the border.

"This undoubtedly represents a verbal escalation," said French military historian Michel Goya.

"But in this arm wrestling match with Russia, you can't stop at anything, this is a poker game."

Alexander Gabuev, director of the Carnegie Russia Eurasia Center, also said Macron's comments represented a toughening of his stance.

"Perhaps it's a form of strategic ambiguity to push Putin to negotiate on Ukrainian terms," Gabuev told AFP.

He pointed out that a small number of Western troops and volunteers were already on the ground in Ukraine, adding that he could not imagine French society backing the idea of sending a large French contingent to fight there.

French Defence Minister Sebastien Lecornu said Macron's statements were not synonymous with escalation.

"To say that we're not ruling anything out is neither being weak nor escalatory," he said.

A number of countries including Germany, Britain and Spain rushed to say they had no plans to send their soldiers to Ukraine.

The Kremlin on Tuesday grimly warned Europe against even entertaining such an idea, saying that the appearance of NATO troops in Ukraine would make a direct confrontation with Russia not only possible but inevitable.

In his early-morning address on February 24, 2022 Putin bluntly warned the West against intervening in Russia's war against Ukraine, and Western countries have been keen to avoid an escalation.

But observers note that over time many red lines have been crossed over the Ukraine conflict, pointing to supplies of French and British long-range cruise missiles, considered unthinkable two years ago.

Kyiv says that Western military support is inadequate, and two years into resisting the full-scale invasion by its much larger neighbour, Ukraine's troops are outgunned and exhausted.

- 'Ramping up nuclear rhetoric' -

Some observers pointed out that Macron's announcement would only help reinforce the Kremlin's narrative that Russia is fighting for survival against NATO-backed Kyiv troops in Ukraine.

Fyodor Lukyanov, head of the Kremlin-linked Council on Foreign and Defense Policy, a think tank in Moscow, said Macron's remarks "will surprise few people in Russia."

Since the start of Moscow's invasion, Putin has made thinly veiled threats hinting at a willingness to deploy Russia's tactical nuclear weapons, which Russian military doctrine holds can be used to force an adversary to retreat.

Lukyanov said that the toughening of France's position would provoke the Kremlin to act tougher.

"This will give Russia an impetus to toughen its stance further, ramp up its nuclear rhetoric and increase reliance on nuclear deterrence as a means of response," he told AFP.

© Agence France-Presse