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.
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Elsewhere, fighters opposed to the government in Moscow say they have captured some Russian soldiers in Belgorod, near the border with Ukraine.
The claim was made by the Liberty of Russia Legion (FRL), which described the announcement as a joint statement with the Russian Volunteer Corps (RDK).
Both groups want to topple President Vladimir Putin. They oppose the full-scale invasion of Ukraine that he launched in February last year.
Belgorod's top official, Vyacheslav Gladkov, replied to say he had agreed to meet the men's captors if the soldiers were still alive. But the fighters later said that the governor "had not found the courage" to meet them and they would hand over their captives to Ukraine.
Russia has blamed Ukraine for recent attacks in its border territories, but Kyiv denies being directly involved.
Authorities in Belgorod said an energy plant was ablaze following a drone attack on Monday morning.
And in Russia's Kaluga region - which borders the southern districts around Moscow - governor Vladislav Shapsha said two drones fell onto a main road. Mr Shapsha said there had not been an explosion and the area was now cordoned off.
There has been no independent confirmation of either attack, but Moscow says the Belgorod region has been the regular target of drone attacks from Ukraine.
Source: BBC
BDST: 1329 HRS, JUN 05, 2023
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