UK senior commanders and Whitehall officials are on "high alert" and monitoring the situation "with growing concern", a source told the Mirror.
Moscow has amassed close to 100,000 troops at the flashpoint border and senior Ukraine military sources fear its war-planners may have wrong-footed them.
Putin’s navy on Thursday piled on further pressure on embattled Kiev by announcing the deployment of a war-fighting fleet towards Ukraine arriving within days.
A source said: “Aggressive Russian troop movements towards the border have caused a great deal of concern within the Ministry of Defence.
Tens of thousands of Ukrainian troops – including reservists- have been called up to the border and US advisers are believed to be “in country,” according to sources.
But western nations and Ukraine have voiced alarm over the build-up as a five strong alliance or “QUINT” of the UK, US, Canada, Lithuania and Poland allies backed Ukraine.
Russia’s Black Sea fleet is based in Crimea, which Russia illegally annexed from Ukraine in 2014. »