Russian officers on Monday (March 10) lauded a daring operation in which hundreds of soldiers crept 16 km (10 miles) along a major gas pipeline and then laid in silent wait for days behind enemy lines in a bid to eject Ukrainian forces from western Russia.

Russia is trying to encircle thousands of Ukrainian soldiers in Kursk just as Ukraine prepares for talks with top U.S. diplomats in Saudi Arabia and U.S. President Donald Trump presses for a swift end to the war.

News of what has been dubbed "operation gas pipeline" or "operation flow" in Russian military circles emerged on Sunday from Russian war bloggers.

Videos showed soldiers in combat paint crammed into a 1.5 metre wide pipe - almost Trojan Horse style - congratulating women on the March 8 International Women's Day, while others used rich colloquial Russian curses.

The date and location of the footage and images shot inside the pipe were not fully verifiable. However soldiers in the footage mentioned they were heading towards Sudzha and referenced March 8.

Handout footage from the Ukrainian military, which was verified to 5km (3 miles) northeast of Sudzha, also showed the targeting of what they said were Russian soldiers. 

Reuters was also able to identify one of the soldiers seen in the video as a Russian soldier.

Russian commanders said the ruse, which led Russian forces behind Ukrainian lines into the town of Sudzha, sowed panic among Ukrainian units and led to a string of Ukrainian defeats in the region, undermining the entire Ukrainian operation.

Some of the Russian soldiers perished, according to Russian sources. Reuters was unable to verify the reports due to reporting restrictions on both sides of the front line.

Ukraine's airborne assault forces said that Russian soldiers had used the pipeline in an attempt to gain a foothold, but the Russians were promptly detected and attacked with rockets, artillery and drones.

Russian forces advanced further in Kursk region on Monday and curled behind Ukrainian forces as part of a major encirclement operation aimed at forcing thousands of Ukrainian soldiers to either flee or surrender in western Russia.

'Operation Pipeline' was planned in secret for weeks and used one of the most enduring remnants of the Soviet Union's energy infrastructure - the vast gas pipelines which used to bring gas to Europe from the wilds of Siberia.

To send men along the pipeline, the gas had to be pumped out and then oxygen pumped in, along with food, water and places for so many soldiers to go to the lavatory.

The 800 soldiers, drawn from a range of Russian units fighting in Kursk, crept along the pipeline and then waited for four days for the command to attack Ukrainian soldiers holding the industrial zone in Sudzha which then fell to Russia.

Ukrainian troops seized about 1,300 square km (500 square miles) of Russia's Kursk region in August in what Kyiv said was an attempt to gain a bargaining chip in future negotiations and to force Russia to shift forces from eastern Ukraine.

Major General Apti Alaudinov, commander of Chechnya's Akhmat special forces and deputy head of the Russian defence ministry's military-political department, praised soldiers before they entered the pipeline.