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.

Ukraine's defence intelligence agency earlier this week said more than 400,000 Russian soldiers were fighting in the country.

Western intelligence said Russia committed around 150,000 troops in its initial assault on Ukraine last February.