Ukraine has launched a fresh offensive in Russia's Kursk region, said the Russian defence ministry.
In a statement, the ministry said efforts to destroy the Ukrainian attack groups were ongoing. Officials in Ukraine have also suggested an operation is under way.
Ukraine first launched its incursion into Russia's Kursk region in August last year, seizing a chunk of territory.
In recent months, Russian forces have made big gains in the area, pushing the Ukrainians back, but failing to eject them entirely.
In a statement posted on Telegram on Sunday, Russia's defence ministry said: "At around 9am Moscow time, in order to stop the offensive by the Russian troops in the Kursk direction, the enemy launched a counter-attack by an assault detachment consisting of two tanks, one counter-obstacle vehicle, and 12 armoured fighting vehicles."
Several Russian military bloggers gave more details about the attack, saying it was launched from the Ukrainians' base at Sudzha towards the villages of Berdin and Bolshoye Soldatskoye, a district centre on the way to Kursk city.
The head of Ukraine's presidential office, Andriy Yermak, said there "was good news from Kursk Region" and that Russia was "getting what it deserves".
Ukraine's top counter-disinformation official Andriy Kovalenko said in a Telegram post on Sunday: "The Russians in Kursk are experiencing great anxiety because they were attacked from several directions and it came as a surprise to them."
It's unclear whether the offensive is sufficiently large-scale to lead to any significant changes on the frontline.
Russian blogger Yury Podolyaka said the operation may have been diversionary, while another, Alexander Kots, did not rule out that the main attack could be launched somewhere else.
Kyiv's forces are reportedly suffering from manpower shortages and have been losing ground in the east of Ukraine in recent months, as Russian troops advance.
It comes as the Ukrainian Air Force said Russia launched another drone attack on Ukraine overnight.
It said it had shot down 61 drones over Kyiv, Poltava, Sumy, Kharkiv, Chernihiv, Cherkasy, Dnipropetrovsk, Zhytomyr, and Khmelnytskyy regions
There were no direct hits, but a few houses were damaged in Kharkiv Region by an intercepted drone, the air force said.
Bd-pratidin English/Lutful Hoque
