The United States (US) on Friday, 11 February, warned of a "very distinct possibility" of a Russian invasion of Ukraine in the near future.
The warning included the likelihood of a potentially overwhelming attack on the Ukrainian capital, Kyiv.
US citizens have been asked to leave the country.
President Joe Biden is scheduled to talk to his Russian counterpart Vladimir Putin on the phone on Saturday, 11 February.
National Security Advisor Jake Sullivan said that while there isn't much to suggest that a final decision has been made, "the way that he (Putin) has built up his forces and put them in place, along with the other indicators that we have collected through intelligence, makes it clear to us that there is a very distinct possibility that Russia will choose to act militarily, and there is reason to believe that that could happen on a reasonably swift timeframe."
The US has promised to hit Russia with massive sanctions in the case of an invasion.
US intel had initially assessed that Putin will wait for the Winter Olympics in China to get over before he decides to launch an assault using the Russian troops that have been mobilised on the Ukrainian border.
The Winter Olympics end on 20 February.
New intel and the latest troop deployments, however, have prompted US officials to say that the invasion could happen before that date as well.
Some US officials, on the condition of anonymity, told the New York Times that Russia is considering Wednesday, 16 February, as the possible date to commence military action.
They also cautioned that the mention of a particular date could be a disinformation scheme initiated by Russia.
Describing a potential attack, Sullivan said that the invasion will probably being with aerial bombing, The Guardian reported.
"If a Russian attack on Ukraine proceeds, it is likely to begin with aerial bombing and missile attacks that could obviously kill civilians without regard to their nationality. A subsequent ground invasion would involve the onslaught of a massive force with virtually no notice. Communications to arrange a departure could be severed and commercial transit halted."Jake Sullivan, National Security Advisor
More than 100,000 troops have been amassed at the Ukrainian border.
Tensions have been simmering for months between Ukraine, NATO, and the US on one side and Russia on the other.
Russia has three demands from NATO:
The removal of NATO troops and infrastructure from Eastern European countries that joined after 1997
A guarantee that NATO would never welcome Ukraine into the alliance
A guarantee to not install missile systems on Russia’s borders
Both NATO and the US have categorically rejected these demands.
A detailed understanding of the background of the tensions can be found here.
(With inputs from NYT and The Guardian.)