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 kilometres 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 Defence Ministry said on Saturday that it believed Ukrainian troops had advanced as much as 50 kilometres 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.

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The Institute for the Study of War, a Washington-based think tank, likewise referenced sweeping Ukrainian gains, estimating that Kyiv has seized around 2500 square kilometres in its eastern breakthrough. The institute 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 Defence 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 defence will shake, and it will fall.”

The Ukrainian military was more circumspect, claiming to have taken “more than 1000 square kilometres” from pro-Kremlin forces this week. It said that “in some areas, units of the Defence Forces have penetrated the enemy’s defences to a depth of 50 kilometres,” 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 on Friday that troops had reclaimed more than 30 settlements in the Kharkiv region since the start of the counteroffensive.

Elsewhere, Ukrainian emergency services reported that a 62-year-old woman was killed in a Russian missile strike in the Kharkiv region when her home was flattened overnight.

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The Ukrainian governor of Kharkiv, Oleh Syniehubov, accused Moscow of pummelling retaken settlements. He said via Telegram that five civilians were hospitalised in the Izyum district, while nine others suffered injuries elsewhere in the region.

In the embattled Donbas, 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. Pavlo Kyrylenko said on Telegram that two people died and two were injured in Bakhmut and the neighbouring village of Yahidne.

In the Russian-held city of Enerhodar, home to Europe’s largest nuclear power plant, electricity and water were restored after a four-day outage due to an explosion, the city’s Ukrainian mayor, Dmytro Orlov, said.

Enerhodar and its Zaporizhzhia Nuclear Power Plant have come under repeated shelling in recent weeks, which Russia and Ukraine accused each other of committing. The shelling has raised fears of a radiation leak at the plant, which has been cut off from outside power sources; the facility has been forced to rely on power from its only working reactor for systems cooling and other safety measures.

Orlov said workers from the plant assisted in restoring Enerhodar’s power, but it was not clear if the electricity was coming from the plant or from a nearby thermal generating station.

Also on Saturday, German Foreign Minister Annalena Baerbock paid an unannounced visit to Kyiv and said Europe would not tire of helping Ukraine, despite Russian President Vladimir Putin’s efforts to raise the pressure by withholding energy supplies.

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.

Despite Ukraine’s gains, US Secretary of State Antony Blinken and the head of NATO warned on Friday that the war would likely drag on for months. Blinken said the conflict was entering a critical period and urged Ukraine’s Western backers to keep up their support through what could be a difficult winter.

AP