The images shared by Kremlin-controlled media were shocking: Russian troops hunched, dirt on their faces, as they crept through an empty gas pipeline under Ukrainian defense lines.

Since Kyiv launched its audacious cross-border assault into the Kursk region last August, Moscow has been pushing back hard, using unconventional tactics and deploying thousands of North Korean troops against the Ukrainian army.

They have since stepped up their advances.

In the past five days, Moscow has broken through Kyiv's defensive lines, reclaiming dozens of square kilometers of territory, according to military bloggers.

Russian military bloggers reported Tuesday that Moscow's troops had entered the town of Sudzha, the largest settlement in the region under Kyiv's control.

"The enemy is retreating in panic and disorder without [having] received any orders. That's it. It's a collapse," a Russian serviceman, who identified himself as Zombie, told Kremlin-run television.

The result is that Kyiv may have lost one of its only bargaining chips on swapping land with Russia, which has seized and occupied around a fifth of Ukraine since it took Crimea in 2014 and launched its full-scale invasion of Ukraine in February 2022.

For Ukraine, which has painted a more controlled picture of the fighting, the stakes of its difficult operation in Kursk could hardly be higher.

The assault last summer injected a much-needed morale boost into the Ukrainian war effort and represented the first and only incursion by a foreign army into Russian territory since World War II.

President Volodymyr Zelenskyy said the operation was key to future negotiations on ending the war and that holding Russian territory would give Kyiv vital leverage.

But that leverage — just as Washington starts rounds of talks with Moscow and Kyiv — is dwindling as Russian forces press forward.

Franz-Stefan Gady, a military analyst, said Russia had built up its force in Kursk over recent weeks and escalated strikes on Ukraine's supply route.

"The result is that now that the Russians are pushing a lot. Parts of the front line are actually giving way," he told AFP.

The British defense ministry estimates Ukraine controls around 300 square kilometers of Kursk, a five-fold territorial loss since Kyiv launched its gambit.

On Monday, Ukrainian commander-in-chief Oleksandr Syrsky conceded the situation was worsening.

He dismissed reports Ukrainian troops were in danger of being encircled but acknowledged they had been forced to fall back and that he was sending forward reinforcements.

One Ukrainian soldier who had been deployed in Kursk told AFP on Monday his unit had "fortunately" withdrawn five days earlier and described fighting there as "very" hard.

Another, who had overseen operations from inside Ukraine and also asked not to be identified to speak freely about the dynamics of the fighting, said Russia's use of drones to disrupt logistics was a key problem.

"It was the end, so to speak. And we started to get out of there because if we didn't, we would have been surrounded," he said, recounting the decision of some troops to leave Kursk because of resupply problems and Russian advances.

From the outset, analysts were skeptical of the purpose of diverting thousands of Ukrainian troops and key military assets from front lines inside Ukraine that were under immense Russian pressure.

With Moscow now clawing back land, this question remains.

Russia last week even claimed to have captured a village inside Ukraine, Novenke, which lies just several kilometers from the vital Ukrainian resupply route into Kursk.

The Institute for the Study of War has said that Moscow is consolidating its gains and likely preparing to attack the largest town still under Ukrainian control, Sudzha.