The US moved its Ukrainian embassy operations from Kyiv to the western city of Lviv due to the threat of Russian invasion as Western leaders pushed for a diplomatic solution to the crisis.
President Joe Biden and UK PM Boris Johnson agreed in a phone call on Monday night that there "remained a crucial window for diplomacy," Downing Street said in a statement.
Western leaders have issued repeated warnings to Russia that any military aggression in Ukraine would result in serious consequences.
The warnings come after Russia amassed more than 100,000 troops on Ukraine's border and made several security demands to the North Atlantic Treaty Alliance (NATO) including that it does not expand to Ukraine or any other former Soviet countries.
It also wants NATO to halt weapon deployments to Ukraine and roll back its forces in Eastern Europe.
"We are basing our assessment on what we are seeing on the ground with our own eyes, which is a continued and unprovoked Russian build-up on the border with Ukraine and no accompanying evidence of de-escalation," said Ned Price, the US State Department spokesperson, on Monday.
"It is a distinct possibility -- perhaps more real than ever before -- that Russia may decide to proceed with military action, with new Russian forces continuing to arrive at the Ukrainian border and Russian forces staged all around Ukraine, an invasion, as we have said, could begin at any time."
Russia has denied plans to invade its neighbouring country and signalled that it was open to continued talks with NATO.
Foreign Minister Sergei Lavrov told President Vladimir Putin on Monday that Moscow should hold more talks with the US and its allies despite their refusal to consider Russia's main security demands.
US Secretary of State
Read more on euronews.com