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…

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

A comrade of the two men, who asked not to be named, told The Telegraph said that they had been captured after running into a much larger Russian force during a battle last Thursday.

“We were out on a mission and the whole thing went absolutely crazy, with bad intel,” he said. “We were told the town was clear when it turned out the Russians were already assaulting it.