Russia is close to completing preparations for what appears to be a large-scale invasion of Ukraine that could result in up to 50,000 civilian deaths, decapitate the government in Kyiv within two days, and launch a humanitarian crisis with up to 5 million refugees fleeing the resulting chaos, according to updated U.S. military and intelligence assessments briefed to lawmakers and European partners over the past several days.

The rising concerns come as the Russian military continues to dispatch combat units to the Ukrainian border in both its own territory and Belarus. As of Friday, seven people familiar with the assessments said, there were 83 Russian battalion tactical groups, with about 750 troops each, arrayed for a possible assault. That is up from 60 two weeks ago, and comprises about 70 percent of what Russian President Vladimir Putin needs to have in place if he wants to maximize the operation.

Those more than 62,000 troops are backed by tens of thousands of additional personnel to provide logistics, air power and medical support. U.S. officials have said the Russian presence along Ukraine’s borders totals more than 100,000; one Western security official put the number at 130,000.

Russia has long bristled over Ukrainian independence. Ukraine was part of the now defunct Soviet Union and for centuries considered part of greater Russia. Ukraine also aspires to NATO membership, which Putin adamantly rejects.

Key military enablers, including bridge-building units, have continued to arrive on the border, and more battalion tactical groups are now in transit, with only a few in far-flung locations, such as the Arctic, remaining at their home bases. As a result, U.S. officials initially skeptical last fall that a large-scale invasion would be launched appear now to have shifted their thinking as the buildup continues, a congressional aide said.

The assessments, the people familiar with them confirmed, also judged that the window for a diplomatic resolution of the crisis appears to be closing. Even as a steady stream of European leaders have been in contact with Putin, further meetings have been scheduled, and the Kremlin has repeatedly denied any invasion plans, the number and configuration of troop movements have continued to push the West’s consensus in the opposite direction.

“Our worry would be that you don’t park battle groups … on the border of another country twice and do nothing,” one European official said, referring to an earlier buildup last year. “I think that’s the real fear that I have. [Putin’s] now put them all out there. If he does nothing again … what does that say to the wider international community about the might of Russia?”

The European official and others familiar with the assessments spoke on the condition of anonymity about intelligence matters.

The new assessments surfaced as Putin was reinforcing his own diplomatic support network. After a meeting Friday with Chinese President Xi Jinping at the opening of the Winter Olympics in Beijing, the two leaders issued a lengthy communique affirming their mutual grievances about the U.S.-led international order, from NATO expansion to security alliances in the Asia-Pacific region. As the United States and its allies have threatened stiff sanctions that could cripple Russia’s oil and gas exports, among other things, Moscow and Beijing agreed to new energy cooperation via a Russian gas pipeline into China.

While not underplaying the significance of the Putin-Xi meeting, and the level of alignment between the two, U.S. officials said that the failure to mention Ukraine in the communique was an indication of China’s general uneasiness about military interventions and instability.

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.

Russian Foreign Ministry Spokesperson Maria Zakharova said that undercutting the Beijing games was one of the reasons the United States and its partners were now spreading what she said was false information.

“As soon as there are talks about a country which is not part of this ‘Western circle’ hosting the Olympic Games … situations surrounding everything become tense immediately – human rights, national interests, regional conflicts and many more,” Zakharova said in a radio interview, according to Russia’s Interfax news agency.

Recent U.S. allegations that Russia was considering staging and videotaping a “false flag” attack purportedly by Ukrainian forces against Russian territory or Russian-speaking people in Ukraine as a pretext for invasion drew fierce denials from Moscow.

On Friday, the Russian Embassy in Washington released a transcript of an exchange between Ambassador Anatoly Antonov and Newsweek in which the diplomat said the United States was making up its own pretexts for war to be used as an “alibi” for a possible Western-backed military operation in Ukraine’s contested Donbas region, where Moscow-backed separatists have been locked in a conflict with Ukrainian government forces for eight years.

“This lie is part of the information war against Russia,” Antonov said of the false flag allegation. “Washington has been provoking the whole world for several months with statements that Ukraine is about to become a victim of ‘Russian aggression.’ ”

Also on Friday, the Russian Foreign Ministry attacked Western leaders such as British Prime Minister Boris Johnson for issuing such statements, saying that they “provoke acrid laughter and jokes,” and are “impossible” to take seriously, the Russian news agency Tass reported.

Some Ukrainian officials, including President Volodymyr 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.

U.S. officials are increasingly concerned that a Russian invasion, at the scale they now believe is indicated, would have widespread global repercussions even if Russian troops do not move beyond Ukraine. What could become the largest military land offensive in Europe since World War II would probably pose broad challenges to the U.S.-led postwar international order of the last 75 years.

As they have watched the assembling of Russian forces north of Ukraine in Belarus, as well as along the Russian border itself, the people familiar with the information said, U.S. officials believe the Kremlin may be positioning them to launch an assault of Kyiv itself by sending troops south to the Ukrainian city of Zhytomyr and moving east toward the capital, while a larger force advances westward from Russian territory.

Such a move would allow the Russians to avoid the site of the Chernobyl nuclear disaster, which lies in Ukraine near the Belarus border.

Satellite imagery has indicated that some Russian ground units already in the region are moving closer to the Ukrainian border. The Russian Defense Ministry said Saturday that Su-255M aircraft, the most advanced version of the jets designed to attack ground targets and low-speed airborne targets, have been deployed to Belarus to take part in ongoing “exercises.”

Russia also has 20 to 30 combat ships in the Black Sea, and could launch amphibious assaults along the coast.

Putin is still not believed to have made a final decision to invade or how far to go, the people familiar with the intelligence said. He could still opt for a smaller invasion along the Black Sea Coast – heading north from the Russian-annexed Ukrainian territory of Crimea – or into the Donbas.

Western officials are divided on whether he would attempt a full-scale or partial invasion. One Western security official said that a full invasion intended to hold territory indefinitely would probably be challenging for Moscow. All signs are that the people of Ukraine would not accept a Russian-installed puppet government and would form a strong resistance movement, both popular and military.

The size of the buildup makes it clear it’s more than a bluff, but some European officials are still not sure, a Western security official said. But the official cautioned that Putin is putting so much political and economic pressure on Ukraine, including by cutting gas transit through its territory, that the government could fall even without a full invasion.

While many believe an assault could be launched any day, optimal conditions are believed to come between mid-February and the end of March, when Ukraine’s flat, open terrain and the rivers cross-crossing it are frozen and armored vehicles can maneuver easily.

One possibility is that Putin may delay until after the Olympics conclude Feb. 20, in order not to upset China by overshadowing the games and threaten Chinese financial assistance in response to U.S. sanctions.

While Ukraine is not part of NATO, and direct military action is not contemplated as a U.S. and allied response to an invasion, a Russian assault is sure to trigger alarm on NATO’s eastern flank, including in Poland and the Baltic states of Lithuania, Latvia and Estonia.

The Pentagon announced Thursday that it would deploy about 3,000 additional U.S. troops to Europe in response to the crisis, including 1,700 to Poland. An initial wave of 300 troops from Fort Bragg arrived Saturday in Wiesbaden, Germany, to activate a new headquarters to oversee the Pentagon response to Russia’s buildup. They named it Combined Joint Task Force Dragon.

The deployments mark a fraction of the 85,000 U.S. troops already in Europe, either on multiyear assignments or shorter rotational deployments. About 1,000 troops already in Germany are being sent to Romania. The new moves are meant to reassure allies and show that an expanded Russian invasion into a NATO ally would trigger a response, and the administration has not ruled out sending other troops already stationed in Europe farther east.

In the event of an invasion, the United States may be forced to rapidly consider what to do about American citizens and U.S. troops who are still in Ukraine.

U.S. officials said that about 7,500 American citizens there have registered with the U.S. Embassy in Kyiv, and there are probably several thousand others who have not done so. For weeks, Americans have been asked to consider leaving, and it remains unclear whether the United States would be able to run any kind of evacuation operation while a Russian invasion also is underway. But unlike Afghanistan, which necessitated an air evacuation last year, Ukraine has western land borders with four NATO countries: Poland, Slovakia, Hungary and Romania.

The United States also still has about 300 U.S. troops in Ukraine, mostly military advisers from the Florida National Guard. Pentagon officials have said they could quickly be withdrawn.