Ukrainian forces have admitted that the Russian army has crossed into the eastern industrial region of Dnipropetrovsk and is trying to establish a base there.
"This is the first attack of such a large scale in the Dnipropetrovsk region," Viktor Trehubov of the Dniprosk Forces Operational-Strategic Group told the BBC, although he claimed that the aggressor's advance had been halted.
Russia has claimed throughout the summer that its forces have entered the area as they try to penetrate deeper into Ukrainian territory from the Donetsk region.
In early June, Russian officials said an offensive had begun in Dnipropetrovsk, although Ukrainian reports suggested they had barely breached the region's border.
Any Russian advance in the Dnipropetrovsk region would be a blow to the morale of Ukrainians, as US diplomatic efforts to end the war appear to be failing, despite US President Donald Trump's meeting with Russian leader Vladimir Putin in Alaska.
The Ukrainian expert group Deep State, which is considered close to the Ukrainian government, estimated on Tuesday that Russia has now captured two villages within the region, Zaporizhzhia and Novohrihorivka.
However, the General Staff of the Armed Forces of Ukraine denied this information. The army "continues to control" Zaporizhzhia, the statement said, and "active fighting is also ongoing in the area of the village of Novohrihorivka".
Moscow has not previously claimed Dnipropetrovsk, unlike Donetsk and four other eastern regions of Ukraine, but has regularly carried out deadly drone and missile attacks on key cities there, including Dnipro, the region's capital.
Before Russia's invasion in February 2022, about three million people lived in the Dnipropetrovsk region, which borders the Donetsk region, including another million in Dnipro.