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The pair were taken prisoner during a fierce battle outside the north-east city of Kharkiv last week, according to comrades who were fighting alongside them.

Alexander Drueke, 39, and Andy Huynh, 27, had been serving as volunteers with a regular Ukrainian army unit. They are believed to be the first US servicemen to end up as Russian prisoners of war.

They will join a growing number of Western military volunteers captured by Russian forces, including three Britons - Aiden Aslin, Shaun Pinner and Andrew Hill.

Last week, a court in the pro-Russian breakaway state of the Donetsk People's Republic sentenced Mr Aslin and Mr Pinner to death, after trying them as "mercenaries".

The capture of the two Americans will be diplomatically sensitive, as the Kremlin may seek to use it as proof that the US is becoming directly involved in the war. Vladimir Putin, the Russian president, is likely to demand significant concessions in exchange for their release.

A comrade of the two men, who asked not to be named, told The Telegraph they had been captured after running into a much larger Russian force during a battle last Thursday in the village of Izbytske, 30 miles north-east of Kharkiv. The village is less than five miles from the Russian border.