Russian forces have thwarted a major Ukrainian offensive in the southern Ukrainian region of Donetsk and killed hundreds of pro-Kyiv troops, the defence ministry in Moscow said on Monday.
In a statement, the ministry said Ukraine had launched the attack on Sunday using six mechanised and two tank battalions.
"On the morning of June 4, the enemy launched a large-scale offensive in five sectors of the front in the South Donetsk direction," the ministry said on the Telegram messaging app.
Reuters was unable to immediately verify the Russian statement.
There was no immediate comment from Ukraine.
It was unclear whether the reported attack represented the start of a long-expected Ukrainian counteroffensive to recapture some of the territory taken by Russian forces after the invasion of February 2022.
"The enemy's goal was to break through our defences in the most vulnerable, in its opinion, sector of the front," the defence ministry said. "The enemy did not achieve its tasks, it had no success."
The ministry also released video of what it said showed several Ukrainian armoured vehicles in a field blowing up after being hit.
Russian forces killed 250 Ukrainian troops as well as destroying 16 tanks, the infantry fighting vehicles and 21 armoured combat vehicles, the ministry said.
Ukraine has in recent weeks sought to weaken Russian positions. Late last month a senior official said preliminary operations such as destroying supply lines or blowing up depots had already begun.
A drone attack set an energy facility on fire in Russia's Belgorod region in the early hours of Monday, the region's governor said. The region borders Ukraine and has been shelled frequently in recent weeks.
On Sunday, Kyiv urged "silence" ahead of the long-expected counteroffensive to reclaim territory that Russia has taken control over in the 15-month long war its been waging on its neighbour.
In an interview published a day earlier, Ukrainian President Volodymyr Zelenskiy said Kyiv was ready to act. Zelenskiy made no mention of the assault in his regular evening video address on Sunday night.
Russian Chief of the General Staff Valery Gerasimov, who is in charge of Moscow's military operation in Ukraine, was in the area of the Ukrainian attack, the ministry said.
"(Gerasimov) was at one of the advanced command posts," the ministry said.
Gerasimov and Defence Minister Sergei Shoigu have been lambasted in recent months by powerful mercenary leader Yevgeny Prigozhin, whose Wagner troops spearheaded the successful assault on the eastern Ukrainian city of Bakhmut last month.
Prigozhin has variously accused top Russian military officials of incompetence and treason. Neither Shoigu nor Gerasimov have responded in public to Prigozhin's insults.