U.S. says Russia has amassed about 75% of the troops it would need to invade Ukraine

The situation along the Ukrainian border has grown even more dire, according to top U.S. officials, who warned Sunday that a Russian invasion of Ukraine may now only be a matter of time.

Speaking on ABC's This Week, National Security Adviser Jake Sullivan offered one of the administration's most stark assessments of the situation, saying that Russian President Vladimir Putin "has put himself in a position with military deployments to be able to act aggressively against Ukraine at any time now."

"We believe that there is a very distinct possibility that Vladimir Putin will order an attack on Ukraine," Sullivan said. "It could take a number of different forms. It could happen as soon as tomorrow or it could take some weeks yet."

With more than 100,000 Russian forces estimated to be in position along Ukraine's borders, U.S. officials believe Putin has already amassed nearly three-quarters of the total amount of troops he would need for a full-scale attack. Russian forces continue to assemble around Ukraine, the officials said, including naval assault ships that could be used to invade Ukrainian ports.

The officials spoke on condition of anonymity to discuss the estimates.

The U.S. estimates that an attack could result in the deaths of tens of thousands of soldiers and civilians alike, while millions of Ukrainian refugees would be left to flee, likely to nearby Poland.

Russian officials have sought to downplay the warnings from Washington, with the country's deputy ambassador to the United Nations, Dmitry Polyanskiy, calling them "madness and scaremongering."

In a tweet, Polyanskiy said, "what if we would say that US could seize London in a week and cause 300K civilian deaths?"

Amid the war of words, some officials continue to hold out for a diplomatic solution. French President Emmanuel Macron is due in Moscow on Monday, and then the Ukrainian capital of Kyiv on Tuesday.

"It is our vital interest to find a solution to this current crisis through dialogue and through engagement, and this is what we're doing currently," said Peter Stano, the lead spokesperson for foreign affairs and security policy for the European Union, in an interview with NPR's Weekend Edition Sunday.

Stano described a possible invasion as a "worst-case scenario" that the EU is prepared for, but hopes will not come to pass.

"We stand united, the European Union [and] our trans-Atlantic partners, including the United States, in facing this challenge because this is one of the most serious challenges in the post-Cold War era and it's an unprecedented challenge to the European Security Order," Stano said. "We will not back down. We will stand firm in support of Ukraine and in defending the international principles which apply here."

Stano went on to call Russia's decision to send an abundance of troops to the Ukrainian border "unprovoked," "unjustified" and "unwarranted."

Russia stands to face heavy sanctions should the situation deteriorate, officials have previously stated. Stano echoed these sentiments during Sunday's interview, explaining that the consequences for Russia would be "much, much tougher" this time than what they've faced in the past.

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