Russia does not say how many troops it has lost during the 20-month offensive, but independent estimates put the numbers well into the tens of thousands.

Moscow has launched an aggressive recruitment campaign, offering huge salaries and welfare programmes in a bid to recruit soldiers for its assault.

"The rate of recruitment for contracted military service has increased significantly. More than 1,600 people are signing a contract with the armed forces every day," Russia's former leader Dmitry Medvedev, deputy chair of the nation's Security Council, said in a video published on social media.

Amid a shortage of manpower, Russia drafted more than 300,000 reservists in a controversial "partial mobilisation" drive last year.

Speculation about a new round of forced call-ups has remained rife as the conflict in eastern Ukraine approaches a winter stalemate and losses continue to mount.