forces said Saturday they had entered Kupiansk in eastern Ukraine dislodging troops from a key logistics hub in a lightning counter-offensive that has seen swathes of territory recaptured.
The German foreign minister, Annalena Baerbock, meanwhile arrived in the Ukrainian capital for a surprise visit, which she said was to demonstrate Berlin's for Ukraine in its battle against Russia.
Ukrainian special forces published images on social media showing camouflage-clad officers with automatic weapons "in Kupiansk". It "was and will always be Ukrainian," their statement said.
The town of some 27,000 people, that sits on a crucial supply route for Russia forces in the east, fell within the first week after the Kremlin ordered its invasion of Ukraine on February 24.
Observers of the conflict expect Ukrainian forces to announce further gains in the Kharkiv region, which borders Russia, and has been either controlled by Russia or shelled by its artillery for months.
There was no official confirmation that Kyiv's troops had also routed Russian forces from Iyzum -- an important staging ground for Russia's war effort -- with a pre-war population of around 45,000 people.
But images flooding social media appeared to show Ukrainian forces within the city and Russian observers of the conflict said there were initial reports Moscow's army had already withdrawn.
"Ukrainian troops are advancing in eastern Ukraine, liberating more cities and villages. Their courage coupled with Western military support brings astonishing results," foreign ministry spokesman Oleg Nikolenko said in a statement on social media.
"It's crucial to keep sending arms to Ukraine. Defeating Russia on the battlefield means winning peace in Ukraine," he added.
His assessment of the pace of the Ukrainian gains came after President Volodymyr Zelensky announced late Friday that his troops had retaken some 30 towns and villages in the northeastern Kharkiv region as part of the sweep.
Ukraine's push appears to have caught Russian troops largely off guard.
The head of the Russian-installed administration of the Izyum told state news agency RIA Novosti that the situation in the region was "very difficult".
"For the past two weeks, the city has been targeted by bombardments by Ukrainian forces... which is causing serious destruction and causing many deaths and injuries," Vladislav Sokolov told RIA Novosti.
Moscow on Friday made the surprise announcement it was dispatching reinforcements to Kharkiv, with images on state media showing tanks and artillery and support vehicles moving in columns on dirt roads.