KYIV, Ukraine — Ukrainian forces claimed Thursday to have shot down a Russian attack helicopter in eastern Ukraine near the city of Avdiivka, where soldiers are fighting from street to street as Russia’s army steps up its four-month campaign to surround Kyiv’s defending troops.

Ukrainian soldiers used a portable anti-aircraft missile to take down the Ka-52 Alligator attack helicopter, one of the Russian air force’s deadliest weapons, according to Oleksandr Tarnavskyi, the commander of Ukrainian units fighting on the southeastern front line.

The roughly 1,500-kilometer (930-mile) line of contact has shifted little during recent months of wintry weather. But as the war in Ukraine nears its two-year anniversary, Avdiivka has become “a primary focus” of Moscow’s forces, the U.K. Defense Ministry said in an assessment Thursday.

Street-to-street combat is taking place in the city as Ukrainian troops seek to keep open their main supply route amid intense bombardment, the ministry said on X, formerly known as Twitter.

The General Staff of Ukraine’s armed forces reported Thursday that its troops had fended off 40 enemy assaults around Avdiivka over the previous 24 hours. That is roughly double the number of daily Russian assaults at other points along the front line.

Russia’s Pravda newspaper reported Thursday that the Russian army was attempting to cut a key logistics supply route for Ukraine in the village of Lastochkyne, about 6 kilometers (4 miles) west of Avdiivka.

The Russian military has used electronic warfare to take out the Starlink communications system which Ukrainian troops use to communicate, Pravda said.

Ukraine has built multiple defenses in Avdiivka, complete with concrete fortifications and a network of tunnels. Despite massive losses of personnel and equipment, Russian troops have slowly advanced since October.

The fight has evolved into a gruesome effort for both sides. It has been compared to the nine months of fighting for Bakhmut, the Ukraine war’s longest and bloodiest battle. It ended with Russia capturing the bombed-out, deserted city last May in what Moscow hailed as a major triumph.

Both Bakhmut and Avdiivka are located in Ukraine’s Donetsk region. Moscow-backed rebels seized part of the region in 2014 and Russia illegally annexed all of it in 2022 with three other Ukrainian regions.

Russia wants to capture the entire Donetsk region, where it currently holds just over half of the territory.

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