'Do you want France to fight with Russia?' Putin asks

And if Ukraine joined Nato, then Nato would be obliged to reconquer Crimea under its Article V mutual defence clause, Putin told French media after meeting Macron in Moscow.

"Imagine that Ukraine is a member of Nato ... This means that there will be a military confrontation between Russia and Nato," Putin said.

"Do you want to fight with Russia?. You ask your readers, viewers, users of Internet resources: 'Do you want France to fight with Russia?'. But that's the way it will be," he added.

Meanwhile on Monday, in another sign tensions could be intensifying, pro-Russian fighters in Russia-occupied east Ukraine called for the Russian army to send reinforcements.

The developments cast doubts over Macron's mission to make Europe's voice count for more amid its biggest security crisis since the Cold War.

Earlier Monday, Putin and Macron talked for six hours about Russia's security demands.

Russia has called on Nato to stop taking in new members and for the US to pull out troops from existing ones, such as Poland and the Baltic states. Russia made the demands after building up what appears to be a large invasion force.

Macron suggested that French leadership was vital in efforts to diffuse the crisis.

"This is France's vocation ... our role is to carry the voice of the European Union and take into account various difficult circumstances in dealing with neighbours such as Russia," Macron said after meeting Putin.

"We have different views, we need to understand and accept this," Macron said of Putin.

The French leader said Nato could not close its doors to potential new members, but mentioned Switzerland and Finland as being free to join Nato as examples rather than directly addressing the question of Ukraine's membership.

"We are ready to work on security guarantees, to build a new order of security and stability in Europe," Macron noted.

Macron will visit Kyiv on Tuesday, while the new German chancellor, Olaf Scholz, will shortly travel to Moscow in a flurry of European diplomacy on the crisis, which included a visit by the Hungarian prime minister to Moscow and phone calls between the Russian, German, Italian, and British leaders.

A noble mission

Scholz also met with US president Joe Biden in Washington on Monday.

"If Russia invades [Ukraine] … again then there will be no Nord Stream 2. We will bring an end to it," Biden said after the meeting, referring to a contested pipeline that is to deliver Russian gas to Germany.

Over the course of his Moscow trip, Macron and Putin made some warm overtures.

Macron spoke of "our neighbour and friend, Russia" and of follow-up phone calls between Paris and Moscow in the coming days.

Putin spoke of the French leader's "noble mission" of coming to Moscow. "He [Macron] is here and has been tormenting me for six hours in a row with questions, guarantees, and solutions," Putin added in a joke.

But the Russian president also inveighed against the expansion of Nato and the alliance's track record in Iraq, Libya, and Serbia.

Putin reiterated that the Russian force of around 125,000 soldiers on Ukraine's border posed no threat.

Putin claimed instead that Ukraine was violating a ceasefire deal called the Minsk Accords, stoking concern that Russia was building a case for intervention.

A related development was the call made by commanders in Russian-occupied regions in east Ukraine asking for Russian aid against alleged Ukrainian abuses.

"We do not rule out that we will be forced to turn to Russia if Ukraine, with the support of Western countries, passes a certain line," Denis Pushilin, the head of the Donetsk People's Republic, told Reuters in an interview on the front line.

Russia should send 30,000 soldiers to the region, Alexander Khodakovsky, a pro-Russian commander, told Reuters in a second interview.