According to the Russian defense ministry, it has stopped a significant Ukrainian offensive and killed 250 Ukrainian soldiers.
Kyiv has not responded, and a third party has not independently verified the Russian claim.
According to the ministry, Ukraine used six mechanized and two tank battalions to begin its offensive in the Donetsk region on Sunday.
Although a Ukrainian counteroffensive has been promised, Kyiv on Sunday requested silence prior to the action.
It is not yet clear whether the alleged attacks point to the beginning of the offensive to retake Ukrainian territory from Russian forces.
The enemy began a massive offensive in five sectors of the front on June 4 in the direction of South Donetsk, according to a Telegram post from the Russian defense ministry.
According to the ministry, the Ukrainians attempted to breach Russian defenses in what Kiev considered to be the most exposed area of the frontline.
"The enemy failed to succeed in completing its objectives. ".
Military vehicles were seen in video being attacked from the air. Moscow asserted that Ukraine had lost 16 tanks and 250 soldiers.
For months, Ukraine has been preparing a counterattack. However, it has sought as much time as possible to prepare its troops and receive military supplies from allies in the West.
The public speculating about the offensive, according to Kyiv officials, could aid the enemy.
Plans adore stillness. No start date will be announced, the defense ministry stated in a video published to Telegram on Sunday.
Armed and masked soldiers were seen holding their fingers to their lips in the video.
There is a lot on the line because the Ukrainian government needs to demonstrate to its citizens that it can cross Russian defenses, end the military standoff, and retake some of its sovereign territory.
Other fighters hostile to the Moscow government claim to have taken some Russian soldiers in Belgorod, close to the Ukrainian border.
The Russian Volunteer Corps (RDK) and the Liberty of Russia Legion (FRL) jointly released the statement, according to the FRL.
President Vladimir Putin's ouster is desired by both parties. They reject his massive invasion of Ukraine, which began in February of last year.
Vyacheslav Gladkov, the top official in Belgorod, responded by stating that if the soldiers were still alive, he had agreed to meet with their captors. Later, however, the fighters claimed that the governor "had not found the courage" to meet with them and that they intended to turn over their hostages to Ukraine.
Recent attacks in Russia's border regions have been attributed to Ukraine, but Kiev disputes any involvement.
Later, Mr. Gladkov claimed that a drone attack had caused a Belgorod energy plant to catch fire.
Although there has been no independent confirmation of the attack, Moscow claims that Ukraine has routinely used drones to attack the region.