Russia's defence ministry says it has thwarted a major Ukrainian offensive in Donetsk, claiming to have killed 250 troops and destroyed armoured vehicles.
Video of what Russia says is the battle appears to show military vehicles coming under heavy fire in fields.
But there has been no comment from Kyiv and Russia's claim has not been independently verified.
A Ukrainian counter-offensive has been long awaited but Kyiv says it will not give advance warning of its start.
And it is unclear whether the alleged attacks indicate that the fresh push to recapture Ukrainian land from Russian forces has begun in earnest.
The Russian defence ministry said Ukraine had launched the "large-scale offensive" in the Donetsk region on Sunday using six mechanised and two tank battalions.
It claimed the Ukrainians tried to break through Russian defences in what Kyiv saw as the most vulnerable part of the frontline but that it "did not achieve its tasks, it had no success".
Moscow claimed Ukraine had lost 250 troops as well as 16 tanks.
Ukraine counter-offensive 'ready to begin'
Ukraine has been planning a counter-offensive for months. But it has wanted as much time as possible to train troops and to receive military equipment from Western allies.
Officials in Kyiv have warned against public speculation over the offensive, saying it could help the enemy.
"Plans love silence. There will be no announcement of the start," the defence ministry said in a video posted to Telegram on Sunday.
The footage featured masked and well-armed troops holding their fingers against their lips.
Much is at stake because the government in Kyiv needs to show the people of Ukraine - and Western allies - that it can break through Russian lines, end the effective military deadlock and recapture some of its sovereign territory.
On Monday morning, the commander of Ukraine's ground forces, Oleksandr Syrskyi, said troops were "moving forward" towards Bakhmut and had destroyed a Russian position near the city.