Ukrainian forces said Saturday they had entered Kupiansk in eastern Ukraine, dislodging Russian troops from a key logistics hub in a lightning counter-offensive that has seen swathes of territory recaptured.
A pro-Russia separatist leader called fighting "difficult" in the Donetsk region while Moscow insisted its forces were "regrouping" after Ukraine's apparent battlefield gains.
It "was and will always be Ukrainian," their statement said.
The town of some 27,000 people, which 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.
Denis Pushilin, the rebel official, said the situation in the town of Lyman in the Donetsk region was "very difficult" and that there was also fighting in "a number of other localities," particularly in the northern part of the region.
- 'Astonishing' advance -
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forces within the city and Russian observers of the conflict said there were initial reports Moscow's army had already withdrawn.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.
Russia's defence ministry said on Saturday that its forces were "regrouping" in an effort "to achieve the goals of the special military operation to liberate Donbas".
This came after 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.
- 'Frightening' -
The capture of urban hubs like Kupiansk and Izyum would be a significant blow to Russia's ability to effectively resupply positions on the eastern frontline and could see Russia pushed back from Kharkiv entirely.
In one village captured by the advancing Ukrainians, electric pylons were toppled and cables lay across the ground and houses were gutted, AFP journalists reported.
"It was frightening," said 61-year-old Anatoli Vasiliev recalling the battle earlier this week that saw Ukrainian forces recapture the village from the Russians.
"There were bombings and explosions everywhere."
Ukrainian troops were also advancing along portions of the southern front line, a spokesperson said Saturday, in some regions by dozens of kilometres, into territory captured by Russian troops at the beginning of the invasion.
Russian news agencies meanwhile reported six large explosions in Nova Kakhovka, a town held by Russian troops in the southern Kherson region.
- 'We will stand by Ukraine' -
German Foreign Minister Annalena Baerbock arrived in the Ukrainian capital on Saturday for a surprise visit, which she said was to demonstrate Berlin's support for Ukraine in its battle against Russia.
It came a week after Ukrainian Prime Minister Denys Shmygal's trip to Berlin where he had repeated Kyiv's call for weapons.
She pledged continued "deliveries of weapons, and with humanitarian and financial support".
Over recent weeks Germany has sent an array of weapons to Kyiv, supplementing other Western-supplied weapons that observers say have hurt Russia's supply and command abilities.
Baerbock's visit follows US Secretary of State Antony Blinken's, during which he promised a nearly $3 billion military package for Ukraine.
In a meeting in Brussels with NATO Secretary General Jens Stoltenberg, Blinken said Russia's push to send reinforcements showed Moscow was paying "huge costs" in its bid to capture and then hold Ukrainian territory.
However, Russian forces were still inflicting serious damage with a campaign of shelling in Kharkiv city and in the industrial region of Donbas in the east.
The head of the Kharkiv region, Oleg Synegubov, said Russian shelling had left 14 civilians injured.
Pavlo Kyrylenko, the head of the Donetsk region, which is part of Donbas, said Russian shelling had left two dead.