Russia's military said Monday it had repelled "a large-scale offensive" by Ukrainian forces in Moscow-annexed Donetsk, as fighting intensified along their border.
Kyiv has for months said it is preparing a major counteroffensive, hoping to reclaim territory lost since Russia launched its military operation in February 2022.
Its army said recently there would be no announcement on the start of the maneuvers.
Russia's Ministry of Defense reported that on Sunday, in the south of Donetsk, "the enemy launched a large-scale offensive in five sectors of the front."
"A total of six mechanized and two tank battalions of the enemy were involved," it said in a Telegram post, adding that Ukrainian troops had hit "the most vulnerable, in their opinion, sector of the front."
"The enemy did not achieve their tasks, they had no success."
The ministry posted what it said was a video of the battle, showing Ukrainian armored vehicles coming under heavy fire.
President Vladimir Putin's top commander in Ukraine, Valery Gerasimov, "was at one of the advanced command posts," its statement added.
Large parts of Donetsk have been held by pro-Moscow separatists since 2014. It is one of four eastern Ukrainian territories that Russia formally annexed in September last year, along with Luhansk, Zaporizhzhia and Kherson.
On the other side of the frontier, Russia's army also claimed to have repelled a "sabotage group of Ukrainian terrorists" seeking to cross the border near the village of Novaya Tavolzhanka, in the Belgorod region.
"The enemy was hit by artillery. The enemy scattered and retreated," it said in a statement.
Belgorod's governor said Sunday that fighting was ongoing near Novaya Tavolzhanka, and acknowledged pro-Ukrainian forces had taken Russians prisoner during cross-border clashes.
"A sabotage group came in, there is combat in Novaya Tavolzhanka," Vyacheslav Gladkov said.
"I hope they will all be destroyed."
It was the first time during the more than 15 months of conflict that a Russian official has admitted the capture of prisoners on Russian territory by pro-Ukraine forces.
Fighting around the village follows last month's dramatic armed incursion from Ukraine into the Belgorod region which forced Russia to use its artillery and air force on home soil.
The border breach was claimed by anti-Kremlin Russian nationalists.
A video published by the nationalists and addressing the governor appeared to show three "prisoners" who their captors called "simple soldiers sent by your leadership to this war."
One appeared to have been injured and was on a drip on a hospital bed.
They called on the governor to meet them in a church in Novaya Tavolzhanka on the Orthodox "Trinity" holiday.
Gladkov said he was ready for talks to retrieve "our guys" from the saboteur fighters.
Ukraine has intensively shelled Russian settlements on the border this week, forcing thousands to flee to the regional hub of Belgorod city.
There, AFP saw volunteers handing humanitarian aid to displaced Russians.
Ukraine has consistently not claimed responsibility for attacks on Russian soil, but presidential adviser Mykhaylo Podolyak said Sunday the situation in the border areas "should be viewed as the future of Russia."
With increased attacks on both sides of the border, the Ukraine conflict has escalated in recent weeks.
Ukrainian authorities said Russian shelling Sunday killed two women in the town of Vovchansk, Kharkiv region, near the border with Belgorod.
"As a result of enemy strikes, two civilian women — aged 62 and 74 — died," the regional prosecutor's office said.