Ukrainian officials say soldiers raising flags and posing in front of street signs across part of previously Russian-held territory.
After keeping silent for a day, Russia effectively acknowledged a section of its frontline had crumbled southeast of Ukraine's second-largest city Kharkiv.
"The enemy is being delayed as much as possible, but several settlements have already come under the control of Ukrainian armed formations," Vitaly Ganchev, head of the Russian-backed administration in the Kharkiv region, said on state television.
Ukrainian President Volodymyr Zelensky later said Kyiv's forces had liberated more than 30 settlements in the Kharkiv region so far and that fighting continued in the eastern Donbas region and the south.
Ukrainian President said that fighting continued in the eastern Donbas region and the south.
"Our army, intelligence units and the security services are carrying out active engagements in several operational areas. They are doing so successfully," he said in a video address.
Ganchev had said his administration was trying to evacuate civilians from cities including Izium, Russia's main stronghold and logistics base in the province.
Zelenskiy adviser Oleksiy Arestovych, in a video posted on YouTube, said the Russian defenders in Izium were almost isolated. Citing what he described as reports from the front line, Arestovych said hundreds of Russians had died so far and several hundred more had been taken prisoner.
Reuters was not able to immediately verify his claim.
Russia has taken control of around a fifth of Ukraine since its troops invaded on Feb. 24 in what Moscow calls a "special military operation" to "disarm" Ukraine. The Kyiv government and its Western allies accuse Russia of an imperial-style war of aggression.
The Russian defence ministry released video of military vehicles speeding along a highway, saying they showed reinforcements rushing to defend the area.
Ukrainian officials released videos showing soldiers raising flags and posing in front of street signs in villages and towns across part of previously Russian-held territory.