Putin has huge forces in occupied east Ukraine – which fell under his sway in 2014, and in Crimea which was occupied.

“Obviously, a major operation is being prepared, and other forces will be brought in as well,” Mr Felgenhauer added.

“Everyone is talking about a possible tank invasion from Belgorod through Kharkiv in the direction of Zaporizhia in order to surround Ukrainian forces on the left bank of the Donbass.”

Mr Felgenhauer, 69, has worked as a military analyst for newspapers in Russia for years.