Russia’s defence ministry says it has thwarted a major Ukrainian offensive and killed 250 Ukrainian troops.
There has been no comment from Kyiv and the Russian claim has not been independently verified.
The ministry said Ukraine had launched the offensive in the Donetsk region on Sunday using six mechanised and two tank battalions.
A Ukrainian counter-offensive has been promised, but on Sunday, Kyiv called for silence ahead of the operation.
It is as yet unclear whether the alleged attacks indicate that the offensive to recapture Ukrainian land from Russian forces has started.
“On the morning of 4 June, the enemy launched a large-scale offensive in five sectors of the front in the South Donetsk direction,” the Russian defence ministry said on Telegram.
The ministry said the Ukrainians tried to break through Russian defences in what Kyiv saw as the most vulnerable part of the frontline.
“The enemy did not achieve its tasks, it had no success.”
Video posted showed military vehicles being attacked from the air. Moscow claimed Ukraine had lost 250 troops as well as 16 tanks.
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.
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.
Mr Gladkov later said an energy plant in Belgorod was on fire following a drone attack.
There has been no independent confirmation of the attack, but Moscow says the region has been the regular target of drone attacks from Ukraine.
