During a televised interview, Macron was asked on Wednesday evening whether France would contemplate striking back if Moscow deployed a nuclear weapon.

"Our (nuclear) doctrine is based on what we call the fundamental interests of the nation and they are defined in a very clear way. It is not at all what would be affected if there was a ballistic nuclear attack in Ukraine or the region," he replied.

He added, however, that when it came to deploying nuclear weapons, "the less we talk about it, the better it is".

That differed sharply with recent comments from US President Joe Biden who warned that the world faced the prospect of "Armageddon" for the first time since the Cold War.

National security experts debated Thursday whether Macron should have telegraphed how he would react if Putin turned to nuclear weapons to reverse recent battlefield losses in Ukraine.

"Until now, Western leaders have kept their cool and have refrained from escalations," Bruno Tertrais, an expert on French nuclear doctrine at the Foundation for Strategic Research, a Paris-based think tank, told AFP.

"But there have been two errors: Biden was clumsy in talking about the prospect of Armageddon, a very strong, biblical and terrifying term. On the other hand, saying as Macron did that we would not respond with a nuclear weapon... Was it necessary to state it?"

Tertrais suggested Macron should have maintained ambiguity.

"In 2020, he talked about the French deterrent as having a European dimension," he added.

Francois Heisbourg, a veteran security analyst at the International Institute for Strategic Studies, wrote on Twitter: "The burden of guessing where our vital interests are lies with the aggressor."

He added that post-war president Charles de Gaulle, the architect of the French nuclear programme, did not put "a strict line at our borders".

"Other presidents never said: we will not do this or that. All rather bizarre," he said.

- 'Vital interests' -

Benjamin Haddad, a member of parliament from Macron's party who was formerly at the Washington-based Atlantic Council think tank, defended the president.

"He was asked the question to which he gave the only possible answer, just restating what has been French nuclear doctrine for half a century," he wrote on Twitter.

Experts believe that any use of a nuclear weapon against Ukraine by Russia would risk sparking a reaction from the Western NATO alliance which has taken care to avoid any direct confrontation with Moscow so far.

But response would most likely be a non-nuclear conventional attack on Russian interests.

"Any nuclear attack against Ukraine will create an answer, not a nuclear answer but such a powerful answer from the military side that the Russian army will be annihilated," EU foreign policy chief Josep Borrell said on Thursday.