Former Russian President Dmitry Medvedev. (Photo: AP)

Medvedev, who is deputy chairman of Russia's Security Council, said in a message on his official social media accounts that Russia would be forced to fall back on its own nuclear doctrine in such a scenario.

Ukraine is trying to retake territory which Russia has annexed and declared to be part of its own territory. Kyiv says its forces are making some progress in their drive to retake territory.