Twenty thousand American troops are preparing for their biggest transatlantic military deployment in 25 years.

Exercise Defender Europe 20 is set to last several months and will involve more than 35,000 personnel from across 18 nations, including the UK.

Britain will be the largest non-US contributor to the exercise with 2,500 soldiers.

Major General Andrew Rohling, Deputy Commanding General of the US Army Europe, said: "This is an exercise in defence, this is an exercise in crisis preparation and an exercise in strategic readiness."

The exercise follows Moscow's annexation of Crimea and accusations of involvement in the Ukraine conflict.

But Major General Rohling said: "It has nothing to do with any specific country."