The Ukrainian forces reversed a Russian advance in Severodonetsk and recaptured about 20 percent of the strategic eastern city over the past couple of days, the governor of Luhansk province said.
Moscow, meanwhile, said Ukraine forces were retreating from the city after suffering heavy casualties.
Fierce fighting raged on Saturday in Severodonetsk and Lysychansk – the last two cities still not in full Russian control more than 100 days after Moscow invaded its western neighbour.
Governor Serhiy Haidai said Russian forces suffered severe defeats and were blowing up bridges across the Siverskyi Donets River to prevent Ukraine bringing in military reinforcements and delivering aid to civilians in Severodonetsk.
“Right now our soldiers have pushed them back. They [the Russians] are suffering huge casualties,” Haidai said in a live TV broadcast on Saturday. “The Russian army, as we understand, is throwing all its efforts, all its reserves in that direction.”
Al Jazeera could not independently confirm the claims.
Both sides have suffered punishing losses in street-by-street battles for the Soviet-era factory city, where roads have been riddled with craters and destroyed vehicles.
If Severodonetsk falls, neighbouring Lysychansk would be the last city that Russia needs to capture to have full control of Luhansk province, which along with Donetsk province makes up the Donbas.
The area has become the focus of Russia’s invasion as President Vladimir Putin seeks to rebuild momentum after a failed attempt to take the capital Kyiv.