Efforts to prevent a Russian invasion of Ukraine will intensify this week, as world leaders make a heightened push for a diplomatic solution, even as new U.S. military and intelligence assessments — which estimated Russia could seize Kyiv in days and leave up to 50,000 civilians killed or wounded — suggest that the window for negotiations is closing.

President Joe Biden is set to meet Monday with new German Chancellor Olaf Scholz, who has been accused of not doing enough to respond to Russian aggression along with his European allies. French President Emmanuel Macron, who recently reemphasized France's commitment to its NATO allies, will meet Russian President Vladimir Putin in Moscow on Monday, and then travel to Kyiv on Tuesday to meet with Ukrainian President Volodymyr Zelensky.

"The priority for me on the Ukrainian question is dialogue with Russia and de-escalation," Macron told reporters last week. "I'm very worried by the situation on the ground."

The White House on Sunday said Biden and Macron had spoken by phone to discuss "ongoing diplomatic and deterrence efforts in response to Russia's continued military build-up on Ukraine's borders." Over the weekend, senior Russian officials dismissed new U.S. intelligence reports that Russia could take over Kyiv in days as alarmist and as unlikely as an attack by Washington on London.

"Madness and scaremongering continues. . . . What if we would say that US could seize London in a week and cause 300K civilian deaths?" Russia's deputy ambassador to the United Nations, Dmitry Polyanskiy, tweeted Sunday.

And parliamentary deputy Artem Turov, a member of Putin's United Russia party, accused the United States of disseminating fake information and of "doing everything possible to fan a new conflict."

On Sunday, Zelensky's office maintained that a diplomatic solution was more likely than war.

"An honest assessment of the situation suggests that the chance of finding a diplomatic solution for de-escalation is still substantially higher than the threat of further escalation," Ukrainian presidential adviser Mykhailo Podolyak said in a statement.

However, Biden national security adviser Jake Sullivan on Sunday defended the updated U.S. military and intelligence assessments that lawmakers and European partners were briefed on over the past several days, which were U.S. officials' bleakest appraisal yet of the deteriorating security situation in Ukraine.

"We're in the window where something could happen. That is, a military escalation and invasion of Ukraine could happen at any time," Sullivan said on NBC's "Meet the Press," adding: "We believe that the Russians have put in place the capabilities to mount a significant military operation into Ukraine, and we have been working hard to prepare a response."

Still, as Biden administration officials have done for weeks, Sullivan stressed there remained a "diplomatic path" forward if Putin chose, even as the U.S. prepared for other scenarios.

"President Biden has rallied our allies. He has reinforced and reassured our partners on the eastern flank," Sullivan said. "He has provided material support to the Ukrainians, and he has offered the Russians a diplomatic path if that's what they choose instead, but either way, we are ready, our allies are ready, and we're trying to help the Ukrainian people get ready, as well."

Seven people familiar with the new U.S. intelligence assessments said Putin has 70% of the combat power he needs for an assault that — under the most extreme scenario — could quickly take out the capital, Kyiv, and remove Zelensky, Ukraine's democratically elected president. Such an invasion, they said, could trigger a refugee crisis in Europe as up to 5 million people flee.

As of Friday, 83 Russian battalion tactical groups, with about 750 troops each, were arrayed for a possible assault. That is up from 60 two weeks ago.

The White House has said the United States does not have information that Putin has made a decision to invade. But satellite imagery and other intelligence indicate he has amassed more than 100,000 troops and equipment on the border with Ukraine — one Western security official put the number at 130,000 — potentially positioning for what could become the largest military land offensive in Europe since World War II.

The Biden administration in recent days has also warned that Moscow was considering filming a fake attack against Russian territory or Russian-speaking people by Ukrainian forces as a pretext to invade its neighbor — a claim the Kremlin has strenuously denied.

The Conflict Intelligence Team, a Russian analytical group that uses open-source data to track Russian military movements, reported Sunday that some Russian forces had moved from a base in Yelnya, in Russia's Smolensk region, closer to the Ukrainian border.

According to the CIT, a "massive" Russian base at Yelnya was nearly empty, in what it described as a "dangerous" development. The CIT said this suggested that "one scenario of a Russian attack is a deep thrust south towards Chernihiv and possibly Kyiv."

Chernihiv is a city in northern Ukraine close to the Belarusian border, less than 90 miles north of Kyiv. The group said that transfers of Russian troops to the Russian regions of Crimea, Rostov and Kursk were also worrying.

As the United States moved to strengthen NATO defenses in Eastern Europe, Germany has been reluctant to export arms to Ukraine, much to the consternation of Kyiv. However, Scholz recently indicated that "all options" — including halting the controversial Nord Stream 2 gas pipeline that would deepen Berlin's reliance on Moscow for energy — are on the table for sanctions in case of a Russian invasion. Scholz will meet Putin on Feb. 15.

The pipeline deal has been the focus of debate in Congress over a package of sanctions aimed at Moscow, with Republicans arguing the administration needs to take a tougher stance with Germany on going into business with Moscow.

Sullivan said Sunday that the pipeline project will not survive a Russian invasion of Ukraine.

"If Russia invades Ukraine, one way or another, Nord Stream 2 will not move forward," Sullivan said on "Meet the Press." "And Russia understands that. We are coordinated with our allies in Europe on that and that will be the reality if Russia chooses to move forward."

Meanwhile, the head of the Belarusian Security Council, Alexander Volfovich, said there would be "very large" military maneuvers with Russian forces in southern Belarus in the coming days, in response to tensions between NATO and Russia over Ukraine. Belarusian President Alexander Lukashenko has been playing a key role in Russia's saber-rattling against Ukraine.

U.S. officials are concerned that the massive Russia-Belarus military exercise, set to begin Thursday, could be used as part of a multipronged invasion of Ukraine. The exercise has seen Russian troops and equipment travel more than 6,000 miles to Belarus, and the deployment of advanced missile systems, fighter planes and bombers.

The U.S.-based company Maxar Technologies on Sunday published satellite images from Friday showing deployments of Russian forces in Belarus.

Moscow has denied that it intends to invade Ukraine but has made clear it considers the presence of Western troops and weapons in the former Soviet sphere an unacceptable security threat. Putin has accused the United States and its European allies of ignoring his key demands to bar Ukraine from joining NATO, rule out putting offensive strike weapons on Russia's borders, and roll back NATO's weaponry and force posture to its 1997 boundaries.

U.S. and other leaders have said Putin's demands to bar Ukraine from joining NATO are a nonstarter, but they have remained open to discussing other security concerns. On Sunday afternoon, Biden was asked by reporters what factors Putin was considering in making his decision.

"I think things he cannot get," Biden said.

Russia also has attempted to paint Ukraine as the aggressor in the crisis, warning that a NATO-backed Kyiv could try to take back Crimea, which Moscow annexed in 2014. Meanwhile, some Ukrainian officials, including Zelensky, have taken issue with Washington's description of Russian deployments and the likelihood of an "imminent" attack, fearing it will cause panic and hurt Ukraine's economy.

Volfovich accused Ukraine of threatening Belarus in comments reported by the BelTA state news agency Sunday. "We did not consider the southern direction as a threat to the country's security before, but today, based on the assessment of the military-political strategic situation, we are forced to consider the southern direction as well," he said.

Volfovich's comments came after 2,000 U.S. troops arrived in Poland and Germany on Sunday to bolster European security.

Although U.S. officials say they believe an assault could be launched any day, optimal conditions are thought to come between mid-February and the end of March, when Ukraine's flat, open terrain and the rivers crisscrossing it are frozen, and armored vehicles can maneuver easily.

One possibility is that Putin may hold off until after the Winter Olympics in Beijing conclude Feb. 20, in order not to upset China by overshadowing the Games and threatening Chinese financial assistance in response to U.S. sanctions.

Putin, meanwhile, has been reinforcing his own diplomatic support network. After a meeting Friday with Chinese President Xi Jinping at the opening of the Olympics, the two leaders issued a lengthy communique affirming their mutual grievances over global issues, including NATO expansion and security alliances in the Asia-Pacific region.

The Biden administration has tried to play down the significance of Putin's outreach to Xi.

A Russian invasion of Ukraine could "embarrass Beijing," because "it suggests that China is willing to tolerate or tacitly support Russia's efforts to coerce Ukraine," Daniel Kritenbrink, the top U.S. diplomat for East Asia, told reporters Friday in a briefing ahead of the visit.

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The Washington Post's Dan Lamothe and Shane Harris in Washington and Amy Cheng in Seoul contributed to this report.

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