Russia said on Monday that its forces had thwarted a major Ukrainian offensive at five points along the front in the southern Ukrainian region of Donetsk and killed hundreds of pro-Kyiv troops.
It was not immediately clear whether or not the reported attack represented the start of a Ukrainian counteroffensive which Kyiv has been promising for months to recapture territory taken by Russian forces after the invasion of February 2022.
Reuters was unable to immediately verify the Russian statement and the Ukrainian defence ministry and military did not immediately respond to written requests for comment.
Kyiv on Sunday urged "silence" ahead of its long-awaited counteroffensive.
Russia's defence ministry said Ukraine had attacked with six mechanised and two tank battalions in southern Donetsk, where Moscow has long suspected Ukraine would seek to drive a wedge through Russian-controlled territory.
"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 defence ministry said in a statement posted on Telegram at 1:30 a.m. Moscow time (2230 GMT).
"The enemy's goal was to break through our defences in the most vulnerable, in its opinion, sector of the front," it said. "The enemy did not achieve its tasks, it had no success."
Russia's defence 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.
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.