LONDON — Russian special forces walked inside a gas pipeline to strike Ukrainian units from the rear inside Russia’s Kursk region, Ukraine’s military and Russian war bloggers reported, as Moscow claimed fresh gains in its push to recapture parts of its border province that Kyiv seized in a shock offensive.

Ukraine launched a daring cross-border incursion into Kursk in August, marking the largest attack on Russian territory since World War II. Within days, Ukrainian units had captured 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 Russian ally North Korea. Tens of thousands of Ukrainian soldiers are at risk of being encircled, open-source maps of the battlefield show.

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

The town had about 5,000 residents before the full-scale February 2022 invasion of Ukraine, and it houses major gas transfer and measuring stations along the pipeline, which was once a major outlet for Russian natural gas exports through Ukraine.

Another war blogger, who uses the alias Two Majors, said fierce fighting was underway for Sudzha, and Russian forces managed to enter the town through a gas pipeline.

Ukraine’s General Staff confirmed 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 Russian troops were “detected in a timely manner” and 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.

The Russian Defense Ministry reported Sunday that its troops had taken four villages north and northwest of Sudzha. Ukraine did not immediately comment on the Russian claims.

Meanwhile, President Donald Trump said in an interview that aired Sunday that Ukraine “may not survive” as he continued to withhold American arms and intelligence in an effort to force Kyiv into peace negotiations with its invader.

Elsewhere, Russian officials and Telegram channels reported that Ukrainian drones targeted oil infrastructure in southern and central Russia overnight into early Sunday.