Russia has stepped up preparations for a full-scale invasion of Ukraine, US officials have said citing intelligence reports.
However, they noted that it was not clear if Moscow had decided to take such a step.
Russia has assembled 110,000 troops along the border with its pro-Western neighbour but US intelligence has not determined if President Vladimir Putin has actually decided to invade, according to the officials who in recent days briefed members of Congress and European allies, AFP reports.
The officials warned lawmakers that the assembled Russian force on the frontier is growing at a rate that would give Putin the force he needs for a full-scale invasion — some 150,000 soldiers –by mid-February, according to France24.
They said Putin wants all possible options at his disposal: from a limited campaign in the pro-Russian Donbas region of Ukraine to a full-scale invasion.
However, Russia has denied it is planning to invade Ukraine.
If Moscow does opt for a full-scale attack, the invading force could take the capital Kyiv and topple President Volodymyr Zelensky in a matter of 48 hours, the officials said.
They estimated such an attack would leave 25,000 to 50,000 civilians dead, along with 5,000 to 25,000 Ukrainian soldiers and 3,000 to 10,000 Russian ones.
It could also trigger a refugee flood of one to five million people, mainly into Poland, the officials added.
President Joe Biden has decided to send American forces to Poland to protect NATO members, as diplomats work furiously to try to persuade Russia to pull its troops back from the border with Ukraine.
The first contingent of US soldiers arrived Saturday.
Russia has also announced what it calls joint military maneuvers with Belarus, where it has sent several battalions to the north of Kyiv and in the Brest region, not far from the border with Poland.
US intelligence has concluded that Russia is continuing to muster a major military force on its border with Ukraine.
Two weeks ago, a total of 60 Russian army battalions were positioned to the north, east and south of Ukraine, particularly in the Crimean peninsula, which Russia annexed after an invasion in 2014.
But on Friday, there were 80 battalions and 14 more were en route from elsewhere in Russia, US officials said.
They added that some 1,500 Russian special forces soldiers known as Spetsnaz arrived along the Ukraine border a week ago.
A major Russian naval force is also positioned in the Black Sea, equipped with five amphibious vessels that could be used to land troops on Ukraine’s southern coast, the US officials said.
They added that another six amphibious craft were observed leaving the Barents Sea north of Russia, sailing past Britain and through the Strait of Gibraltar, apparently on their way to the Black Sea.
In other deployments, Russia has positioned fighter planes near Ukraine, as well as bombers, missile batteries and anti-aircraft batteries, US officials said.
On Thursday, the United States said it had evidence that Russia was preparing a video depicting a bogus attack by Ukraine that would serve as a pretext for a real Russian assault on Ukraine.