Russian special forces walked kilometers (miles) inside of a gas pipeline to strike Ukrainian units from the rear in the Kursk region, Ukraine’s military and Russian war bloggers reported, as Moscow moves to recapture parts of its border province that Kyiv seized in a shock offensive.

Ukraine launched a daring cross-border incursion into Kursk i n August, in what marked the largest attack on Russian territory since World War II. Within days, Ukrainian units had captured 1,000 square kilometers (386 square miles) of territory, including the strategic border town of Sudzha, and taken hundreds of Russian prisoners of war.

According to Kyiv, the operation aimed to gain a bargaining chip in future peace talks, and force Russia to divert troops away from its grinding offensive in eastern Ukraine.

But months after Ukraine’s thunder run, its soldiers in Kursk are weary and bloodied by relentless assaults of more than 50,000 troops, including some from Russia's ally North Korea. Tens of thousands of Ukrainian soldiers run the risk of being encircled, open source maps of the battlefield show.

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According to Telegram posts by a Ukrainian-born, pro-Kremlin blogger, Russian operatives walked about 15 kilometers (9 miles) inside the pipeline, which Moscow had until recently used to send gas to Europe. Some Russian troops had spent several days in the pipe before striking Ukrainian units from the rear near the town of Sudzha, blogger Yuri Podolyaka claimed.

The town had some 5,000 residents before the February 2022 Russian invasion of Ukraine, and houses major gas transfer and measuring stations along the pipeline, once a major outlet for Russian natural gas exports through Ukrainian territory.

Another war blogger, who uses the alias Two Majors, said fierce fighting was underway for Sudzha, and that Russian forces managed to enter the town through a gas pipeline. Russian Telegram channels showed photos of what they said were special forces operatives, wearing gas masks and moving along what looked like the inside of a large pipe.

Ukraine’s General Staff confirmed on Saturday evening that Russian “sabotage and assault groups” used the pipeline in a bid to gain a foothold outside Sudzha. In a Telegram post, it said the Russian troops were “detected in a timely manner” and that Ukraine responded with rockets and artillery.

“At present, Russian special forces are being detected, blocked and destroyed. The enemy’s losses in Sudzha are very high,” the General Staff reported.