KYIV, Ukraine (AP) — Western defense officials and analysts on Saturday said they believed Ukraine had punched through Russian front lines south of the country’s second-largest city, taking thousands of square miles of territory and threatening to cut off Russian supply lines.

The British Defense Ministry in an online briefing said it believed the Ukrainians had advanced as much as 50 kilometers (30 miles) in the advance south of Kharkiv, northeastern Ukraine.

The advance appears 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.

It added that the nearby town of Kupiansk also appeared to be pressured by Ukrainian forces, and that its loss would greatly affect Russian supply lines in the area.

An image circulated on social media Saturday appeared to show Ukrainian soldiers in front of a main government building in Kupiansk, some 73 kilometers (45 miles) north of Izyum.

The image showed soldiers displaying the flag of the 92nd Separate Mechanized Battalion of Ukraine in front of a building that resembled Kupiansk’s city council building, which sits just along the Oskil River.

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.

A Washington-based think tank likewise referenced sweeping Ukrainian gains on Saturday, estimating that Kyiv has seized around 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 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 that Ukrainian forces “may collapse Russian positions around Izyum if they sever Russian ground lines of communication” north and south of the town.

The fighting in the east comes amid an ongoing 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.

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 since the launch of its long-awaited counteroffensive 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 any geographical details.

Officials in Kyiv have for weeks been tight-lipped about their plans for a counter-offensive to retake territory overrun by Russia early in the war, calling on local residents to refrain from sharing information on social media for fear of compromising the ongoing operation.

Moscow did not immediately acknowledge or comment on the claims by Ukraine and its Western allies.

Elsewhere, Ukrainian emergency services reported that a civilian was killed in a Russian missile strike in the Kharkiv region overnight.

The regional branch of the State Emergency Service said that the body of a 62-year-old woman was found buried in the rubble of her home, which was flattened by the strike.

The regional governor, Oleh Syniehubov, also accused Moscow of pummeling settlements retaken 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 district, while nine others suffered injuries elsewhere in the region.

In the embattled Donbas region south of Kharkiv, the Ukrainian governor said that 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, Germany 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 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 by Ukrainian forces.

___ Kozlowska reported from London. Associated Press writer Frank Jordans in Berlin contributed to this report.