Emmanuel Macron has said there is a "very strong unity" between France and Germany in seeking to defuse tensions over the Russia-Ukraine security crisis.
But, he added, "in the event of an attack, the response will be there and the cost (to Russia) will be very high".
The French President was speaking at a joint news conference in Germany with Chancellor Olaf Scholz, on his first visit to the country since last year's election.
"We are preparing simultaneously the common reaction and response in the event of an attack," he stressed, claiming that the West was "totally united", and repeating its joint appeal for a de-escalation of tensions.
Macron confirmed that he will speak by phone with Russian President Vladimir Putin on Friday. The Kremlin has also said a conversation will take place. The French president has said he wants to propose a "road map to de-escalation" in the next few days.
Paris is also due to host a meeting of top advisers from Germany, Russia and Ukraine on Wednesday under the "Normandy format", in an attempt to revive a dialogue that began in 2014 but is all but dead. "To give up is to acknowledge that the political situation is no longer on the table," Macron said, adding that the talks would create "positive expectations".
Questioned on Germany's refusal to send arms to Ukraine, Chancellor Scholz said the country was acting from a point of principle "stemming naturally from the evolution of these past years and decades", a choice linked to its responsibility for the Second World War.
Russia has denied it is planning an assault, but it has massed an estimated 100,000 troops near Ukraine in recent weeks, leading the United States and its NATO allies to rush to prepare for a possible war.
Several rounds of high stakes diplomacy have failed to yield any breakthroughs, and this week tensions escalated further.
Biden: sanctions could target Putin
Washington focused on Tuesday on sanctions that would be levied on Russia — and even on Vladimir Putin himself — in the event of an invasion of Ukraine.
Asked whether he could envisage sanctions targeting the Russian president personally, US President Joe Biden replied "yes".
Meanwhile, the United States and European countries have taken measures to protect European gas supplies if ever Russia decided to cut them off should conflict break out in Ukraine, Washington said on Tuesday.
Senior White House officials indicated that western allies were looking at diversifying supplies from other countries, saying there would be "consequences" for Russia should it seek to "weaponise" the flow of gas.
UK ready to boost NATO troops
British Prime Minister Boris Johnson on Tuesday said that the European security order could not be allowed to be threatened because "Russia has placed a gun to Ukraine's head".
Johnson reaffirmed Britain's readiness "to contribute to any new NATO deployments to protect our allies in Europe" as he addressed parliament.
He added that were Russia to invade Ukraine again, "the resistance will be ferocious".
The latest shipment of US military aid including defensive equipment and munitions arrived in Kyiv on Tuesday, part of a new US$200 million (€177 million) in security assistance directed to Ukraine from the United States.
The US has ordered 8,500 troops on higher alert to potentially deploy to Europe as part of an alliance “response force" if necessary, while NATO said it was bolstering its deterrence in the Baltic Sea region.
The Pentagon said on Tuesday it was not ruling out additional US troop overseas deployments in response to heightened tensions over Russia's military presence.
Russia sends more troops to Belarus
Adding to its large troop deployment near Ukraine, Russia has moved more troops from Siberia and the Far East for joint drills with its ally Belarus.
Russian military units have moved to areas near Belarus' southern border, which is about 75 kilometres from Kyiv. The deployment will further beef up Russian military presence near Ukraine amid Western fears of a planned invasion.
Belarusian leader Alexander Lukashenko said Belarus was strengthening its military forces at the southern border with Ukraine, while the joint manoeuvres with Russia would also be conducted on the country's western border.
Ukraine seeks to reassure nation
Ukraine's leaders sought Tuesday to reassure the nation that an invasion from neighbouring Russia was not imminent, even as they acknowledged the threat is real and received a shipment of US military equipment to shore up their defences.
Authorities in Kyiv have sought to project calm in order not to destabilise the situation and avoid panic — and many citizens have expressed skepticism that there will be an invasion soon.
In parliament, Defence Minister Oleksii Reznikov said that “as of today, there are no grounds to believe” that Russia is preparing to invade imminently, noting that its troops have not formed what he called a battle group that could force its way through the border.
“Don't worry, sleep well,” he said. “No need to have your bags packed.”
Reznikov's remarks follow multiple reassurances from President Volodymyr Zelenskyy and other officials. On Monday, Zelenskyy told the nation that the situation was “under control.”
In an interview aired late Monday, however, the defence minister acknowledged that "there are risky scenarios" that "are possible and probable in the future".