Russia has claimed it thwarted a "major" attack in the Donetsk region and killed hundreds of Ukrainian troops, in what appears to be an unverified statement.
The Kremlin's forces said Ukraine was making a major effort to punch through Russian defensive lines in southeast Ukraine, but failed.
A statement from Russia's defence ministry claimed 250 Ukrainian troops were killed.
Kyiv initially didn't confirm the attacks and suggested the claim was a Russian disinformation scheme.
However, the country's Deputy Defence Minister Hanna Malyar later said Ukrainian forces are "shifting to offensive actions" in some areas.
Vladimir Rogov, a Moscow-installed official in southeast Ukraine’s partly-occupied Zaporizhzhia province, claimed that "the enemy threw an even bigger force into the attack than yesterday."
Rogov said Ukraine's new attempt to break through the front line was "more large-scale and organised", he said, adding: "A battle is underway."
Rogov's comments came after Moscow also claimed to have thwarted large Ukrainian attacks in the eastern Donetsk region — another of the four regions that President Vladimir Putin claimed as Russian territory last autumn.
Anticipation is growing for Ukraine's long-anticipated spring offensive, but no one knows when it will begin.
Today's unverified reports have fuelled speculation that the ground operation could be underway.
The commander of Ukraine's ground forces, Gen Oleksandr Syrskyi, said troops were "moving forward" towards Bakhmut and had destroyed a Russian position near the city but did not say whether it was the same attack Russia has spoken about.
A video published by the Ukrainian Defense Ministry showed soldiers putting a finger to their lips in a sign to keep quiet.
"Plans love silence", it said on the screen.
Oleksii Reznikov, the Ukrainian Defence Minister, published a cryptic message on Twitter on Sunday amid speculation about when the operation is going to begin.
He quoted a Depeche Mode song, saying: "Words are very unnecessary. They can only do harm."
Michael Clark, the former head of the Royal United Services Institute think tank, said the “increased tempo” of activity in recent weeks likely marked the start of the counteroffensive and that June is likely to see the start of Ukraine’s ground operation.
The Russian Defense Ministry said the alleged Donetsk attack started Sunday morning. It was unclear why it waited until early Monday to announce it.
The Russian Defense Ministry spokesman said Ukraine used six mechanised and two tank battalions in the Donetsk attacks.
The ministry released a video claiming to show the destruction of some of the equipment in a field.
