Russia said on Sunday that its troops had taken the village of Basivka in Ukraine's northeastern Sumy region, and were battering Ukrainian forces at a host of settlements in the area.
More than two years after the Russian invasion of Ukraine, Kyiv sent thousands of troops over the border into Russia's Kursk region in August last year though a Russian offensive over recent months has pushed most of Ukrainian forces out of Kursk.
Russian President Vladimir Putin has repeatedly suggested that Russian forces carve out a buffer zone along the border.
Russia's defense ministry said that it had taken the village of Basivka, just over the border from Sudzha, and had struck Ukrainian forces at 12 other points in the Sumy region.
Ukrainian officials later denied the report, saying Russian forces were not in control of Basivka.
"As of today, the Russians do not control Basivka in Sumy region. They are trying to run in there in assault groups and look for cellars in order to gain a foothold, but the enemy is being destroyed," Andriy Kovalenko, an official of Ukraine's National Security and Defense Council, said on Telegram messenger.
"The fighting in the Sumy region border area is complex and continues daily in several areas, and is also taking place in the Kursk border area," he added.
Russia's defense ministry also said that Russia had defeated Ukrainian units in the Russian settlements of Gornal, Guevo and Oleshnya.
The pro-Ukrainian DeepState war map shows Ukraine in control of about 63 square kilometers (24 square miles) of Russian territory, down from as much as 1,400 square kilometers claimed by Kyiv last year.
Another 81 square kilometers of territory along the border - including Basivka - is classed by DeepState as of "unknown" control.
Russia currently controls a little under one fifth of Ukraine, including Crimea which Russia annexed in 2014, and most but not all of four other regions which Moscow now claims are part of Russia - a claim not recognized by most countries.
Russia controls all of Crimea, almost all of Luhansk, and more than 70% of Donetsk, Zaporizhzhia and Kherson regions, according to Russian estimates. It also controls a sliver of Kharkiv region.
