Ukraine's military appeared to have punched through front lines in the country's north-east, taking thousands of square kilometres of territory and threatening to cut off supply lines to occupying Russian forces, western defence officials and analysts said on Saturday
The British Defence Ministry in an online briefing said it believed the Ukrainians had advanced as much as 50 kilometres in a push south of Kharkiv, the country’s second-largest city.
The advance appeared to be around Izyum, long a focus on the Russian front line and the site of heavy artillery and other fighting. The British 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.
Natalia Popova, adviser to the head of the Kharkiv regional council, shared photos on Facebook of troops holding up a Ukrainian flag in front of the city hall in Kupiansk, about 73 kilometres north of Izyum.
"Kupiansk is Ukraine. Glory to the armed forces of Ukraine," she wrote.
With Ukrainian forces having reached Kupiansk, where rail lines linking Russia to eastern Ukraine converge, the advance had penetrated all the way to Moscow's main logistics route, potentially trapping thousands of Russian troops.
Ukraine’s military has not yet acknowledged entering the city, though it comes amid a several days of apparent gains by the Ukrainians south of Kharkiv.
Elsewhere in the Kharkiv region, Reuters journalists reported seeing Ukrainian police patrolling in towns recaptured in recent days and heaps of ammunition abandoned by fleeing Russian soldiers.
The Washington-based Institute for the Study of War estimated on Saturday that Kyiv had seized about 2,500 square kilometres in its north-eastern breakthrough.
It appeared that “disorganised Russian forces [were] caught in the rapid Ukrainian advance”, the think tank said, citing social media images apparently showing Russian prisoners seized in the advance around Izyum and surrounding towns.
Ukrainian forces “may collapse Russian positions around Izyum if they sever Russian ground lines of communication” north and south of the town, it said.
Oleksiy Arestovych, an adviser to Ukraine's presidential office, in a video posted on YouTube, said the Russians in Izyum were almost isolated.
The fighting in the east has come amid an offensive around Kherson in southern Ukraine. 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.
Moscow acknowledged that its frontline buckled in Kharkiv but said it was rushing extra troops to reinforce the area.
The head of the Russian-installed administration in Kharkiv's occupied areas, Vitaliy Ganchev, described the advance as a Ukrainian victory and called for civilians to flee.
The Ukrainian military was more circumspect about the reported gains, claiming in its regular update on Saturday to have taken “more than 1,000 square kilometres” from pro-Kremlin forces since the launch of its long-awaited counteroffensive 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 any geographical details.
In an overnight video address, President Volodymyr Zelenskyy said at least 30 settlements had been liberated in Kharkiv region.
"Our army, intelligence units and the security services are carrying out active engagements in several operational areas. They are doing so successfully," he said.
Officials in Kyiv have for weeks been tight-lipped about their plans for a counter-offensive to retake territory overrun by Russia soon after its forces entered Ukraine on February 24, calling on local residents to refrain from sharing information on social media for fear of compromising the ongoing operation.
Kharkiv's governor, Oleh Syniehubov, accused Moscow of pummeling settlements retaken by Kyiv in its recent advances, along with other residential areas in the region.
Mr Syniehubov said in a Telegram post that five civilians were admitted to hospital in the Izyum district, while nine others suffered injuries elsewhere in Kharkiv.
In the Donbas region south of Kharkiv, the local 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.
Ukraine's attack in the east came as a surprise, only a week after it announced the start of a long-awaited counter-attack to reclaim Russian-occupied territory hundreds of kilometres away at the opposite end of the front in Kherson.
Limited information has been made public about that operation, but Kyiv has also claimed some successes there, cutting supply routes to thousands of Russian troops isolated on the west bank of the Dnipro River.
"We see success in Kherson now, we see some success in Kharkiv and so that is very, very encouraging," US Defence Secretary Lloyd Austin told a news conference in Prague on Thursday.
Secretary of State Antony Blinken visited Kyiv the same day and announced a new $2.8 billion US military aid package for Ukraine.
Meanwhile, Germany's Foreign Minister Annalena Baerbock arrived in Kyiv on an unannounced visit on Saturday, saying Europe would not tire of helping Ukraine despite Russian President Vladimir Putin’s efforts to raise the pressure by withholding energy supplies.
Mr Baerbock said Germany would assist Ukraine in finding and removing mines and other unexploded ordnance left by Russian troops in areas where they were pushed back by Ukrainian forces.
Tens of thousands of people have been killed, millions were driven from their homes and Russian forces have destroyed entire cities since Moscow launched what it calls a "special military operation" to "disarm" Ukraine.
— With reporting from agencies.