The Ukrainian forces are continuing with a counter-offensive in Eastern Ukraine in which they have made significant gains this week. Officials have said the Ukrainian forces have cut vital Russian supplies to Izyum and have also retaken Kupiansk. Izyum was a major base for Russian forces in the Kharkiv region.

Earlier this week, Ukrainian forces also liberated Balakliya town after seven months of Russian occupation. Videos surfaced on the internet showing Ukrainian people greeting Ukrainian soldiers who had driven the Russians out.

"The town of Balakliya, Kharkiv region, is liberated by Ukrainian troops! The Commander of Ukrainian Land Forces, Hero of Ukraine, Colonel General Oleksandr Syrskyi is leading the Ukrainian offensive in this sector. The Ukrainian flag has been raised in the town centre," said Ukraine's Ministry of Defense on Twitter on Friday.

These Ukrainian military advances in the country's east around Kharkiv, Ukraine's second-largest city, could become the biggest battlefield success for Ukrainian forces since they thwarted a Russian attempt to seize the capital Kyiv at the start of the nearly seven-month war.

"The Russian army in these days is demonstrating the best that it can do — showing its back. And, of course, it's a good decision for them to run," said Ukrainian President Volodymyr Zelenskyy in a video on Saturday.

Russian Defence Ministry spokesman Igor Konashenkov said troops would be regrouped from the Balakliya and Izyum areas to the eastern Donetsk region, adding that the Russian move was being made "in order to achieve the stated goals of the special military operation to liberate Donbas" — the Eastern Ukrainian region largely controlled by Russian-backed separatists made up of two provinces named Donetsk and Luhansk.

The claim of a withdrawal to concentrate on Donetsk is similar to the justification Russia gave for pulling back its forces from the Kyiv region earlier this year when they failed to take the capital.

Igor Girkin, a Russian who was an early leader of a Moscow-backed separatist uprising in Donetsk in 2014, sneered at the portrayal of the pullback being strategic. On the messaging app Telegram, he acidly called it "the brilliant (clearly within the framework of the plan and even ahead of schedule) operation to transfer the cities of Izyum, Balakliya and Kupiansk to respected Ukrainian partners".

Earlier Saturday, Ukrainian officials claimed major gains in the Kharkiv region, saying their troops had cut off vital supplies to Izyum.

Ukrainian Foreign Ministry spokesman Oleh Nikolenko also suggested troops had retaken Kupiansk, a town along the main supply route to Izyum, long a focus on the Russian front line and the site of heavy artillery and other fighting. Nikolenko tweeted a photo showing soldiers in front of what he said was a government building in Kupiansk, 73 kilometers (45 miles) north of Izyum.

The Ukrainian Security Service posted a message hours later saying troops were in Kupiansk, further suggesting it had been seized. The military did not immediately confirm entering the town, a railway hub that Russia seized in February.

Videos on social media appeared to show Ukrainian forces on the outskirts of Izyum at a roadside checkpoint. A large statue with the city's name could be seen in the images. Ukrainian forces did not acknowledge holding the city.

Britain's Defense Ministry said Saturday that it believed Ukrainian troops had advanced as much as 50 kilometers south of Kharkiv, and described Russian forces around Izyum as "increasingly isolated".

"Russian forces were likely taken by surprise. The sector was only lightly held and Ukrainian units have captured or surrounded several towns," said the British military, adding that the loss of Kupiansk would greatly affect Russian supply lines.

The Institute for the Study of War, a Washington-based think tank, likewise referenced sweeping Ukrainian gains, estimating that Kyiv has seized around 2,500 square kilometers (965 square miles) in its eastern breakthrough. It said it appeared that "disorganised Russian forces (were) caught in the rapid Ukrainian advance", and cited social media images of apparent Russian prisoners seized around Izyum and surrounding towns.

The same report said Ukrainian forces "may collapse Russian positions around Izyum if they sever Russian ground lines of communication" north and south of the town.

Vladislav Sokolov, head of the Russian-appointed local administration, said on social media that authorities in Izyum had started evacuating residents to Russia.

The fighting in eastern Ukraine comes amid an ongoing offensive around Kherson in the south. Analysts suggest Russia may have taken soldiers from the east to reinforce the latter area, offering the Ukrainians the opportunity to strike a weakened front line.

Ukrainian Defense Minister Oleksii Reznikov told the television channel Ukraina that the Russians had no food or fuel for their troops in the area as Kyiv had cut off their supply lines.

He said, "It will be like an avalanche. One line of defense will shake, and it will fall."

The Ukrainian military was more circumspect, claiming to have taken "more than 1,000 square kilometers" (386 square miles) from pro-Kremlin forces this week. It said that "in some areas, units of the Defense Forces have penetrated the enemy's defenses to a depth of 50 kilometers", matching the British assessment, but did not disclose geographical details.

Officials in Kyiv have for weeks been tight-lipped about plans for a counteroffensive, urging residents to refrain from sharing information on social media.

However, Zelenskyy said Friday that troops had reclaimed more than 30 settlements in the Kharkiv region since the start of the counteroffensive.

The New York Times has termed the Ukrainain advances into the East as "surprise Ukrainian offensive". It reported that the loss of logistical hub Izyum could be "a turning point in the war".

"The Ukrainian offensive has reshaped what was becoming a grinding war of attrition. One village after another has fallen, including the west side of the town of Kupiansk," reported The Times, citing George Barros, an analyst for the Institute for the Study of War.

(With AP inputs)