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, in an attempt to revive a dialogue which began in 2015 but is all but dead.
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, and there would be "consequences" for Russia should it seek to "weaponise" the flow of gas.
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 the Britain's readiness "to contribute to any new NATO
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