The ruse was among moves aimed at cutting off thousands of Ukrainian soldiers in the region ahead of Ukrainian talks with the United States on a possible peace deal to end the war.
Ukrainian troops seized about 1300 square km of Russia's Kursk region in August last year in what Ukraine said was an attempt to gain a bargaining chip in future negotiations and to force Russia to shift forces from eastern Ukraine.
Russia has been pressing its push to regain control of the region with some success in recent days.
Open source maps on Friday showed Ukraine's contingent in Kursk nearly surrounded after rapid Russian advances.
"The lid of the smoking cauldron is almost closed," former Russian president Dmitry Medvedev said on Telegram.
"The offensive continues."
Yuri Podolyaka, a Ukrainian-born pro-Russian military blogger, said Russian special forces crept nearly 16km along the inside of the 1.5 metre wide gas pipeline and spent several days in the pipe before surprising Ukrainian forces from the rear near Sudzha.
Pro-Russian war blogger Two Majors said a major battle was under way for Sudzha and that Russian forces had surprised Ukrainian soldiers by entering the area via a major gas pipeline.
A statement from Ukraine's airborne assault forces said that Russian soldiers had used the pipeline in an attempt to gain a foothold but the Russian troops were promptly detected and attacked with rockets, artillery and drones.
Russian advances in 2024 and US President Donald Trump's upending of US policy on Ukraine and Russia have raised fears among European leaders that Ukraine will lose the war and that Trump is turning his back on the continent.
The United States paused military aid and the sharing of intelligence with Ukraine this month after a meeting between Trump and Ukrainian President Volodymyr Zelenskiy on February 28 descended into acrimony in front of the world's media.
In its daily update on the situation in Kursk, Russia's defence ministry said its forces had retaken the village of Lebedevka, as well as seizing Novenke, a hamlet across the border in Ukraine's neighbouring Sumy region.
Russia made no official mention of the pipeline operation but Major General Apti Alaudinov, commander of Chechnya's Akhmat special forces, reposted pictures on Telegram of special forces inside a gas pipeline.
"I am surprised by people who really think that Russia could lose," Alaudinov said.
"It is a good day."
Russian Telegram channels showed pictures of special forces in gas masks and lights, some using colourful colloquial Russian curses, as they made their way along the inside of what looked like a large pipe.
Owing to battlefield reporting restrictions on both sides, Reuters was unable to verify the reports.
The Soviet-era Urengoy-Pomary-Uzhgorod pipeline used to bring gas from western Siberia via Sudzha to Ukraine but Ukraine terminated all Russian gas transit through its territory from January 1.
Ukraine's invasion of Kursk last August was the most serious attack on Russian territory since the Nazi incursion of the Soviet Union in 1941.
Another war blogger, Yuri Kotenok, said that Ukrainian forces have been moving equipment away from Sudzha, closer to the border.
The Russian offensive raises a serious tactical conundrum for Ukraine just as the spring thaw turns roads to mud tracks: should it withdraw from Kursk, and if so, can it do so without a disorderly rush to the border under intense Russian fire.
In the eastern Donetsk region, where Russian forces have made slow but steady progress during gruelling fighting in what was once Ukraine's industrial heartland, Russia said on Sunday that its forces had taken the village of Konstyantynopil.
