KYIV, Ukraine (AP) -- Ukrainian officials on Saturday claimed major gains in a counteroffensive against Russian forces in the country's northeast, saying Ukrainian troops had cut off vital supplies to front-line hotspot.
The reports followed several days of apparent advances by Ukraine south of Kharkiv, the country's second-largest city, in what could become the biggest battlefield success for Ukrainian forces since they thwarted a Russian attempt to seize the capital of Kyiv at the start of the nearly seven-month war.
Ukrainian Foreign Ministry spokesman Oleh Nikolenko suggested the Ukrainian 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 of the 92nd Separate Mechanized Battalion of Ukraine in front of what he said was a government building in Kupiansk, 73 kilometers (45 miles) north of Izyum.
The Security Service of Ukraine, known as the SBU, hours later posted a message which it said showed its forces in Kupiansk, further suggesting it had been seized by Ukrainian troops. The Ukrainian military didn't immediately confirm entering the town, a railway hub which Russia seized in February, when its mayor surrendered to avoid civilian casualties.
Later Saturday, videos on social media began to circulate also appearing to show Ukrainian forces on the rural outskirts of Izyum at a roadside checkpoint. A large statue bearing the city's name could be seen in the footage. Ukrainian forces did not acknowledge holding the city.
Earlier Saturday, the British Defense Ministry told reporters it believed the Ukrainians had advanced as much as 50 kilometers (30 miles) 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," the British military said, adding that the loss of Kupiansk would greatly affect Russian supply lines in the area.
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 northeastern breakthrough. The institute said it appeared that "disorganized Russian forces (were) caught in the rapid Ukrainian advance." They cited social media images of apparent Russian prisoners seized in the advance 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.
Moscow did not immediately acknowledge or comment on the claims by Ukraine and its Western allies. However, Vladislav Sokolov, the head of the Russian-appointed local administration, said on social media that authorities in Izyum have 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 around Kherson, 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.
"It will be like an avalanche," he said, predicting a Russian fallback. "One line of defense will shake and it will fall."
The Ukrainian military was more circumspect about the reported gains, claiming in its regular update Saturday to have taken "more than 1,000 square kilometers" (386 square miles) from pro-Kremlin forces this week. It said "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 to retake territory overrun by Russia early in the war, calling on residents to refrain from sharing information on social media.
However, Ukrainian President Volodymyr Zelenskyy said in a nightly video address Friday that Ukrainian troops had reclaimed more than 30 settlements in the Kharkiv region since the start of the counteroffensive.
"We are gradually taking control over more settlements, returning the Ukrainian flag and protection for our people," Zelenskyy said.
He spoke after the Ukrainian governor of Kharkiv reported that the national flag had been raised over Balakliia, a town recaptured by Ukrainian troops on Thursday following six months of Russian occupation.
"Balakliia is Ukraine! Today, together with the military, led by the commander of the Ground Forces Oleksandr Syrskyy, we raised the Ukrainian flag," governor Oleh Syniehubov wrote on Telegram.
Elsewhere, Ukrainian emergency services reported that a 62-year-old woman was killed in a Russian missile strike in the Kharkiv region overnight. It said her body was found buried in the rubble of her home, which was flattened by the strike.
Syniehubov also accused Moscow of pummeling settlements retaken by Kyiv in its recent advances, along with other residential areas in the region. He said in a Telegram post that five civilians were hospitalized in the Izyum district, while nine others suffered injuries elsewhere in the region.
In the embattled Donbas region south of Kharkiv, the Ukrainian governor said civilians were killed and wounded overnight by Russian shelling near the city of Bakhmut, a key target of the stalled Russian offensive there. Pavlo Kyrylenko said on Telegram that two people died and two more suffered injuries in Bakhmut and the neighboring village of Yahidne.
Meanwhile, German Foreign Minister Annalena Baerbock arrived in Kyiv on an unannounced visit Saturday, saying that Europe would not tire of helping Ukraine despite Russian President Vladimir Putin's efforts to raise the pressure by withholding energy supplies from European Union nations.
Baerbock said Germany will assist Ukraine in finding and removing mines and other unexploded ordnance left by Russian troops in areas where they have been pushed back by Ukrainian forces.
Despite the gains made by Ukraine's armed forces, U.S. Secretary of State Antony Blinken and the head of NATO warned Friday that Ukraine's fight against Russia appears set to drag on for months. Blinken said the war was entering a critical period and he urged Ukraine's Western backers to keep up their support through what could be a difficult winter.