Russia has massed some 100,000 troops near Ukraine's border and though Moscow says it has no plans to invade its neighbour, President Vladimir Putin has demanded legally-binding guarantees that NATO will not expand further eastwards.
"We will not accept the campaign Russia is waging to subvert its democratic neighbours," Foreign Secretary Liz Truss told parliament. "They have falsely cast Ukraine as a threat to justify their aggressive stance."
"Russia is the aggressor here," Truss said. "NATO has always been a defensive alliance."
Russia annexed the Black Sea peninsula of Crimea from Ukraine in 2014, drawing sanctions and condemnation from the West.
Truss said that any further military incursion into Ukraine by Russia would bring "massive consequences, including coordinated sanctions to impose a severe cost on Russia's interests and economy."
"The UK is working with our partners on these sanctions, including high impact measures targeting the Russian financial sector and individuals," Truss said.
Putin says NATO's expansion eastwards since the 1991 fall of the Soviet Union is a threat to Russia which, he says, has nowhere left to retreat to. He has warned the West against ignoring his concerns.
Truss said she would visit Kyiv later this month and that the situation was reaching a crucial moment with only one way forward: for Putin to step back from the brink.