Russia's military build-up along its border with Ukraine is now sufficiently advanced that President Vladimir Putin could order an invasion "as soon as tomorrow", US National Security Adviser Jake Sullivan said on Sunday.
The warning comes a day after US officials said Russia had moved at least 70 per cent of the military hardware and troops it needed to launch a full-scale invasion of its neighbour, sparking a conflict that could lead to tens of thousands of casualties.
"We believe that there is a very distinct possibility that Vladimir Putin will order an attack on Ukraine," Mr Sullivan told ABC News.
"It could take a number of different forms. It could happen as soon as tomorrow, or it could take some weeks yet."
Mr Putin "has put himself in a position with military deployments to be able to act aggressively against Ukraine at any time now", Mr Sullivan said.
Mr Sullivan reiterated that the US and its allies are prepared to "walk the diplomatic path" to find an exit from the crisis.
Ukrainian Foreign Minister Dmytro Kuleba on Sunday cautioned against such "apocalyptic predictions" about an imminent invasion.
President Volodymyr Zelenskiy said a week ago that Ukraine was not a sinking Titanic and accused Washington and media of fuelling panic that upset the economy when there were "no tanks in the streets".
"Today, Ukraine has a strong army, unprecedented international support and Ukrainians' faith in their country," Mr Kuleba said in a tweet.
"The enemy should be afraid of us, not us of them."
On Sunday, a few dozen elite US troops landed in south-eastern Poland near the border with Ukraine after President Joe Biden’s orders to send 1,700 soldiers there to as part of a larger deployment to support eastern European Nato allies .
Hundreds more infantry troops of the 82nd Airborne Division were still expected to arrive at the Rzeszow-Jasionka airport, 90 kilometres from Poland’s border with Ukraine.
In Warsaw, Polish Defence Minister Mariusz Blaszczak hailed the military assistance, saying “deterrence and solidarity are the best response to Moscow’s aggressive policy, to the aggressive attempt at reconstructing the Russian empire”.
US officials on Saturday said a full-scale invasion could lead to tens of thousands of deaths, although Russian forces might seek a limited military incursion rather than occupying Kiev.
The New York Times and The Washington Post reported that officials gave warnings that a full Russian invasion could lead to the quick capture of Kiev and result in up to 50,000 casualties.
Their assessment matches recent analysis by military experts highlighting Mr Putin’s military build-up, focusing on the movement of blood supplies and engineering equipment.
US officials have detailed indicators suggesting Mr Putin intends an invasion in coming weeks.
An exercise of Russia's strategic nuclear forces that usually is held each autumn was rescheduled for mid-February to March. That coincides with what US officials see as the most likely window for invasion.
The officials made no suggestion that a prospective conflict would involve nuclear weapons, but the Russian exercise — probably involving the test-launching of unarmed long-range missiles on Russian territory — could be used as a message aimed at deterring the West from intervening in Ukraine.
US officials have said in recent weeks that a Russian invasion could overwhelm Ukraine's military relatively quickly, although Moscow might find it difficult to sustain an occupation and cope with an insurgency.
The continuing Russian build-up comes as the Biden administration has been disclosing intelligence in hopes of pre-emptively countering Russian disinformation and blocking Mr Putin's plans for creating a pretext for an invasion.
But it has come under criticism for not providing evidence to back up many of its claims.
Mr Biden has said he will not send US troops to Ukraine to fight a war. He has, however, ordered additional forces, including headquarters personnel and combat troops, to Poland and Romania to reassure Nato allies.
Ukraine is not a Nato member but receives US and allied military support and training.
With growing nervousness in Eastern Europe over Russia's build-up, much attention is focused on its placement of thousands of troops in Belarus, which shares a border with Ukraine and three Nato nations — Poland, Lithuania and Latvia.
The Biden administration may soon shift more troops within Europe to allied nations on Nato's eastern flank, a US official said on Saturday, without specifying which countries.
US Defence Secretary Lloyd Austin said last week that Mr Putin could use any part of the force he has assembled along Ukraine's borders to seize cities and “significant territories”, or to carry out “coercive acts or provocative political acts” such as the recognition of breakaway territories inside Ukraine.
Officials have outlined the disposition of Russian forces that have been sent towards Ukraine's borders over the past several months, creating what western officials see as the threat of a full-scale invasion despite repeated assertions by senior Russian officials that they do not intend to attack unprovoked.
As of Friday, the officials said, the Russian army has put in place near Ukraine a total of 83 “battalion tactical groups”, each of which is about equal in size to an American battalion of between 750 and 1,000 soldiers.
That is an increase from 60 battalion tactical groups in position just two weeks ago, they said.
Another 14 battalion tactical groups are on their way to the border area from other parts of Russia, the officials said.
Two officials said the US assesses that Russia would want a total of between 110 and 130 battalion tactical groups for use in a full-scale invasion, but Mr Putin could decide on a more limited incursion.
Including support units, Russia might be aiming to have 150,000 troops in place for a full-scale invasion, an official said, adding that the continuing build-up could reach that level in the next couple of weeks.
Depending on Mr Putin's ultimate objective, the Russian forces could attack Kiev directly by moving south from current positions in southern Belarus.
He might also send forces across the Russian border into eastern and southern Ukraine if his intent is to fracture and destroy a large part of the Ukrainian army, the officials said.
On the lower end of the scale of military action, Mr Putin might order sabotage, cyber attacks and other destabilising action inside Ukraine with the goal of removing the current government in Kiev, officials have said.
Agencies contributed to this report.