Russian advance troops are already operating in Ukraine ahead of a possible invasion, a defence minister warned today.
James Heappey said a' significant' number of Vladimir Putin's soldiers were already operating behind the lines.
His comments came amid a deepening east/west stand-off and a growing military build-up, with the US readying to send 8,500 combat troops to the region.
An estimated 100,000 Russian troops have massed at the border with Ukraine and intense diplomatic activity has failed to ease tensions.
Denmark is sending a frigate to the Baltic Sea and is set to deploy F-16 fighter jets to Lithuania, Spain is sending ships and is considering sending fighter jets to Bulgaria, France has expressed its readiness to send troops to Romania, and the Netherlands is sending two F-35 fighter planes to Bulgaria from April.
Britain has already supplied Kyiv with thousands of anti-tank weapons and Mr Heappey likened the situation to the Nazi invasion of Poland in 1939 that triggered the Second World War.
Writing in the Sun he said: 'To be British is to go to the aid of others and defend those who cannot defend themselves.
'That's why we went to the aid of Poland in 1939. That's why we were one of the founding allies of Nato after the Second World War ... we are being tested again.'
Russia has been warned it will be swiftly hit with an 'unprecedented package of sanctions' in the event of a fresh incursion in Ukraine after Boris Johnson joined Joe Biden and world leaders to present a united front to combat a feared invasion.
The Kremlin today expressed concern over what it said was an 'exacerbation of tensions' by Western nations.
James Heappey said a' significant' number of Vladimir Putin's soldiers were already operating behind the lines.
A convoy of Russian armored vehicles moves along a highway in Russian-occupied Crimea. An estimated 100,000 troops have massed at the border with Ukraine
Boris Johnson (right) has delivered a stark message to Vladimir Putin (left at the Kremlin last night) as he admitted that the prospects of averting a confrontation in the area were 'gloomy'.
Ukrainian territorial volunteers take part in military training near Kiev at the weekend amid fears Russia could invade
The Prime Minister and the US President met virtually with the leaders of Nato, the EU, Italy, Poland, France and Germany last night as they hoped to avert what Mr Johnson warned would be a bloody and protracted conflict for Moscow.
Downing Street said they stressed diplomatic discussions with Russia remains the first priority, but said the nation would be hit with 'swift retributive responses' if a 'further Russian incursion into Ukraine' takes place.
Mr Heappey said today: 'Where we see Russian plans we will expose them, such as their ''false flag'' attacks and plots for puppet regimes.
'As I write, we are becoming aware of a significant number of individuals that are assessed to be associated with Russian military advance-force operations and currently located in Ukraine.'
A Ukrainian soldier stands at the line of separation from pro-Russian rebels, Donetsk region, as fears grow of an invasion
Ukraine was a part of the Soviet Union until it was disbanded. A young woman walks past a World War II-era Soviet T-34 tank at an outdoor commemorative display extolling the battles and victories of the Soviet Red Army in Kyiv.
Under efforts to deter Russian 'aggression', Mr Biden said the leaders discussed preparations to 'impose severe economic costs' against Moscow while seeking to 'reinforce security on the eastern flank'.
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