Russia announced Saturday it was pulling back troops from the eastern Kharkiv region of Ukraine as Kyiv announced its troops had seized a key logistics hub in a lightning counter-offensive.
A Moscow-backed separatist leader in the east meanwhile said Russian forces were fighting "difficult" battles against Kyiv's troops in several parts of eastern Donetsk region.
A Ukrainian official also said Kyiv's troops were closing in on the eastern city of Lysychansk, captured by Russian troops after fierce artillery battles in July.
Moscow's announcement of the pull back alongside Kyiv's claim to have entered the town of Kupiansk are the most significant shifts in battlefield dynamics after months of fighting in eastern Ukraine that has been dominated by Moscow.
"A decision was made to regroup Russian troops stationed in the Balakliya and Izyum regions to bolster efforts along the Donetsk front," Russia's defence ministry said in a statement.
News of the draw down came just after Ukrainian special forces published images on social media showing camouflage-clad officers with automatic weapons "in Kupiansk".
The town of some 27,000 people -- which was captured with in the first week of Russia's invasion launched on February 24 -- "was and will always be Ukrainian," the special forces said.
But observers expect Ukrainian forces to make further gains in the Kharkiv region, which borders Russia and has been either controlled by Russia or shelled by its artillery for months.
- 'Astonishing' advance -
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forces within the city and Russian observers of the conflict said there were initial reports Moscow's army had already withdrawn.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 came after President Volodymyr Zelensky said late Friday his troops had already retaken some 30 towns and villages in the northeastern Kharkiv region.
Ukraine's push appears to have caught Russian troops largely off guard.
One day before Russia announced its three-day operation to redeploy forces from Kharkiv to the industrial Donbas region further south it said it was dispatching reinforcements to Kharkiv.
State media on Friday published footage of Russian tanks, artillery and support vehicles moving in columns on dirt roads moving towards Kharkiv.
- 'Frightening' -
The capture of urban hubs like Kupiansk and Izyum would be a significant blow to Russia's ability to resupply its positions on the eastern frontline and could see Russia's hold over the east severely diminished.
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.
On the road towards the recaptured town of Balakliya, the journalists saw abandoned Russian armour painted with the letter "Z" -- a symbol of Russia's invasion.
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.
Meanwhile in the eastern Donetsk region, rebel leader Denis Pushilin said the situation in the town of Lyman was "very difficult" and that there was also fighting in "a number of other localities," particularly in the northern part of the 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.
It came a week after Ukrainian Prime Minister Denys Shmygal's trip to Berlin where he had repeated Kyiv's call for weapons.
Baerbock 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 Saturday on the Kholodnogirskiy district of the city had left at least one person dead and two more injured.
Earlier, Pavlo Kyrylenko, the head of the Donetsk region, which is part of Donbas, said Russian shelling had left two dead.
Concerns have also been rising in recent days over fresh shelling near Ukraine's Zaporizhzhia nuclear power plant in the south of the country.
The International Atomic Energy Agency said Friday recent attacks near the plant had compromised the safe operation of the facility.
French President Emmanuel spoke with Zelensky on Saturday and said the situation at the plant was "worrying," according to the Elysee.
The Ukraine leader again said the site should demilitarised during the call, according to a tweet.