"For our soldiers, for all those who are in particularly tough battles these days. We see your heroism, and we are grateful to you for every minute of your life," Zelensky said in his daily address.

Ukrainian officials have said their forces are ready for a counteroffensive to claw back territory in the east and south, but that there would be no formal announcement when it begins.

Earlier, Russian President Vladimir Putin said the counter-offensive had started but that efforts "so far have failed," after Moscow said it repelled several Ukrainian assaults.

As pledged, Ukraine has said little about the clashes.

In his daily address, Zelensky made no direct reference to the rumoured counter-offensive or specific developments on the battlefield.

"We focus our attention on all directions where our actions are needed and where the enemy may suffer certain defeats," he said.