Europe is going through its most dangerous moment since the Cold War amid fears of a Russian invasion of Ukraine, European Union foreign policy chief Josep Borrell said.

“We are living, to my understanding, the most dangerous moment for security in Europe after the end of the Cold War,” Borrell told reporters on Monday.

His comments came as French President Emmanuel Macron told Russian President Vladimir Putin that he aimed to avoid war and build trust during a visit to Moscow.

The trip makes Macron the first top Western leader to visit Moscow since Russia began massing troops on the border with Ukraine.

Macron, who is expected to seek re-election in April, has positioned himself as a potential mediator on Ukraine, with Paris voicing scepticism about predictions by Washington and other Western capitals that a Russian assault is imminent.

Macron told the Russian president he was seeking a “useful” response “that of course allows us to avoid war and to build bricks of trust, stability, visibility”. Putin, for his part, said Russia and France shared “a common concern about what is happening in the security sphere in Europe”.

“I see how much efforts the current leadership of France and the president personally is applying in order to solve the crisis related to providing equal security in Europe for a serious historical perspective,” Putin said.

On the eve of his trip, Macron, who is also due in Kyiv on Tuesday, told the Journal du Dimanche newspaper: “The geopolitical objective of Russia today is clearly not Ukraine, but to clarify the rules of cohabitation with NATO and the EU.”

On his arrival, Macron told reporters: “I’m reasonably optimistic but I don’t believe in spontaneous miracles.”

Al Jazeera’s Dorsa Jabbari, reporting from Moscow, said there was a sense that differences remained after the meeting, noting Putin first appeared “boisterous” to meet with Macron, but after the talks ended, “it was slightly a different story”.

“There wasn’t as much optimism from Vladimir Putin; he was very much sticking to his guns and his point, and some of the things he had to say were very, very, to-the-point and matter-of-fact,” she said.

“There was no sugarcoating any of the things that he feels at the moment. He said that if Ukraine joins NATO, and if Ukraine tries to take Crimea back, there will be a war.”

Kremlin spokesperson Dmitry Peskov had said ahead of the talks: “The situation is too complex to expect decisive breakthroughs in the course of one meeting.”

In Washington, United States President Joe Biden said as he hosted German Chancellor Olaf Scholz that they were “working in lockstep” to address the crisis. Biden said diplomacy is the best way forward for all sides in the standoff.

Russia has deployed more than 100,000 troops near Ukraine’s borders. It denies planning an invasion, but says it is ready to take unspecified “military-technical measures” if its demands are not met, including a promise by NATO never to admit Ukraine and to withdraw some troops from Eastern Europe.

Russia continues to add military forces along its border with Ukraine, the Pentagon said on Monday.

“Even in just over the course of the weekend, we saw Mr Putin add to his force capability along that border with Ukraine and in Belarus,” Pentagon spokesman John Kirby said. “He is well north of 100,000 and it continues to grow.”

‘Guarantees for Russia’

Washington has rejected those demands but says it is willing to talk about arms control and confidence-building steps, which Moscow says are beside the point.

“In recent days there has been nothing new on the topic of security guarantees for Russia. Our Western interlocutors prefer not to mention this topic,” Peskov said.

The US and its allies have ruled out defending Ukraine with military force but say they would respond to any invasion with sanctions, arms shipments and reinforcement of NATO countries nearby.

Last week Biden ordered nearly 3,000 US troops to deploy in Poland and Romania to better protect NATO’s eastern flank. A US general arrived in Poland on Saturday and the bulk of the new forces due there were expected on Monday.

Germany announced on Monday that it would deploy 350 troops to Lithuania to reinforce a NATO battle group there.

Russian-backed separatist leaders in eastern Ukraine warned a full-scale war could break out there and urged Moscow to send 30,000 soldiers to reinforce rebel forces. Kyiv says 15,000 people have been killed in fighting between government forces and separatists in the breakaway Donetsk region since 2014. Russia had annexed Ukraine’s Crimean peninsula that same year.

Meanwhile in London, a spokesman for British Prime Minister Boris Johnson said Russian concerns about potential NATO aggression were “fundamentally unfounded as NATO is a defensive alliance at its heart”. He said the United Kingdom wanted to work with Moscow to provide it with reassurance on that point.

Russia sees NATO’s addition of 14 new East European members since the Cold War ended three decades ago as an encroachment on its sphere of influence and a threat to its security.