Ukraine’s military claims to have broken through Russia’s first line of defence in the occupied Kherson region.
The reported push appears to form part of a long-awaited offensive being launched by Kyiv in an attempt to retake the country’s south.
It follows weeks of Ukrainian attacks aimed at cutting off Russian forces there from main supply routes.
Russia’s military has not commented on Ukraine’s claim, but one official said this was “yet another fake”.
“Ukrainian formations are suffering severe losses both in the south and in all other directions,” said Sergei Aksyonov, the Moscow-installed head of Crimea – Ukraine’s southern peninsula annexed by Russia in 2014.
The claims by both Ukraine and Russia have not been independently verified.
Russia has occupied large swathes of Ukraine’s Kherson region since its invasion began on 24 February.
On Monday, Ukraine’s Kakhovka operational group in the south said that one regiment of Russian-backed forces had left its positions in the Kherson region.
It added that Russian paratroopers providing the back-up had fled the battlefield.
Ukraine’s Hromadske TV said three of its sources in Ukraine’s military confirmed that the first line of defence had been broken.
Meanwhile, Ukrainian army spokeswoman Natalia Humeniuk said “any military operation requires ‘silence’ regime”, urging Ukrainians to be patient.
“Our main efforts have been focused on destroying the enemy’s ammunition storage bases. More than 10 of such bases were destroyed over the past week,” she said.
But Ms Humeniuk refused to give any further details.
In a telegram post the Ukrainian military also claimed on Monday to have struck multiple targets in the region, including a factory in Beryslav and a Russian army post by the North Crimean Canal.