Western defense officials and analysts said Saturday they believed Ukraine had pierced Russian front lines south of the country’s second-largest city, seizing thousands of square miles of land and threatening to cut off Russian supply lines. Britain’s Ministry of Defense said in an online briefing that it believed the Ukrainians had advanced up to 50 kilometers (30 miles) in the advance south of Kharkiv in northeastern Ukraine. The advance appears to be around Izyum, centered on the Russian front and the site of heavy artillery and other fighting. The British described the Russian forces around Izyum as “increasingly isolated”. “Russian forces were likely taken by surprise. The sector was lightly held and Ukrainian units captured or surrounded several towns,” the British military said. He added that the nearby town of Kupiansk also appeared to be under pressure from Ukrainian forces and that its loss would greatly affect Russian supply lines in the region. An image circulated on social media on Saturday appeared to show Ukrainian soldiers in front of a central government building in Kupiansk, about 73 kilometers (45 miles) north of Izyum. The image shows soldiers displaying the flag of Ukraine’s 92nd Separate Mechanized Battalion in front of a building that looked like the Kupiansk City Council building, which is located right along the Oskil River. Ukraine’s military has yet to acknowledge its entry into the city, though it comes amid apparent days of gains by Ukrainians south of Kharkiv. A Washington-based think tank also cited Ukraine’s sweeping gains on Saturday, estimating that Kyiv has seized about 2,500 square kilometers (965 square miles) in its northeastern breakthrough. The Institute for the Study of War said in a report that it appeared “disorganized Russian forces (were) caught up in the rapid Ukrainian advance.” They reported images on social media of what appeared to be Russian prisoners seized during the advance around Izyum and surrounding towns. The same report said Ukrainian forces “may collapse Russian positions around Izyum if they cut Russian land lines of communication” north and south of the city. The fighting in the east comes amid an ongoing offensive around Kherson in southern Ukraine. Analysts suggest Russia may have brought troops from the east to reinforce around Kherson, offering the Ukrainians an opportunity to strike a weakened frontline. The Ukrainian military was more circumspect about reported gains, claiming in its regular briefing on Saturday that it had taken “more than 1,000 square kilometers” (386 square miles) from pro-Kremlin forces since launching its long-awaited counteroffensive this week. He said that “in some areas, units of the Defense Forces have penetrated enemy defenses to a depth of 50 kilometers,” matching the British estimate, but did not reveal any geographic details. Officials in Kyiv have for weeks been tight-lipped on plans for a counteroffensive to recapture territory seized from Russia early in the war, urging local residents to refrain from sharing information on social media for fear of jeopardizing the develop business. Moscow did not immediately acknowledge or comment on the claims by Ukraine and its Western allies. Elsewhere, Ukrainian emergency services said a civilian was killed in a Russian missile strike in the Kharkiv region overnight. The regional department of the State Emergency Service announced that the body of a 62-year-old woman was found buried in the rubble of her home, which was leveled by the strike. The regional governor, Oleh Syniehubov, also accused Moscow of crushing settlements recaptured by Kyiv in its recent advances, along with other residential areas in the region. Syniehubov said in a Telegram post that five civilians were hospitalized in the Izyum region, while nine others were injured elsewhere in the region. In the war-torn Donbass region south of Kharkiv, the Ukrainian governor said civilians were killed and wounded overnight by Russian shelling near the town of Bakhmut, a key target of the Russian offensive there. Pavlo Kyrylenko told Telegram that two people were killed and two others were injured in Bakhmut and the neighboring village of Yahidne. Meanwhile, German Foreign Minister Annalena Baerbock arrived in Kyiv on an unannounced visit, saying Europe would not tire of helping Ukraine despite Russian President Vladimir Putin’s efforts to increase pressure by withholding energy supplies. Baerbock said Germany would help Ukraine find and remove mines and other unexploded ordnance left by Russian troops in areas where they have been pushed back by Ukrainian forces. —— Kozlowska reported from London. Associated Press writer Frank Jordans in Berlin contributed to this report.