Britain warned Moscow on Thursday that it was working with Western partners on high-impact sanctions targeting Russia's financial sector should it invade Ukraine.

Russia has massed some 100,000 troops near Ukraine's border and though Moscow says it has no plans to invade its neighbor, 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 neighbors," Foreign Secretary Liz Truss told Parliament. "They have falsely cast Ukraine as a threat to justify their aggressive stance."