The United States has placed 8,500 troops on "heightened alert" over the Ukraine standoff, warning that Russia has "no intention right now of de-escalating".
Pentagon press secretary John Kirby said no final decisions have been made on deployments and that the order is about ensuring that the US is ready to respond if Nato decides to deploy its response force.
UK Prime Minister Boris Johnson was on a call with US President Joe Biden and a number of key European leaders about the escalating situation.
Earlier on Monday, Mr Johnson said that a Russian invasion of Ukraine would be "painful, violent and bloody business" for Vladimir Putin, as Britain withdrew its diplomats from Kyiv.
The Prime Minister said the UK was "leading on creating a package of economic sanctions" against Russia but added: "We also need to get a message that invading Ukraine, from a Russian perspective, is going to be a painful, violent and bloody business."
"I think it's very important that people in Russia understand that this could be a new Chechnya," he said.
Nato allies have already sent aircraft and warships to Eastern Europe.
