Moscow controls around one-fifth of Ukraine's territory, but has captured just one percent of the country over the past year, according to an AFP analysis of data from the Institute of the Study of War, which works with the Critical Threats Project.

Despite the slow progress, fighting on the front line has remained intense, with both sides sustaining immense losses.

Ukrainian commander-in-chief Oleksandr Syrsky said Russia had switched its frontline tactics and was now deploying small groups of four-to-six soldiers tasked with infiltrating Ukraine's defences.

"Since the beginning of summer, there has been a change in the enemy's tactics," Syrsky told a group of journalists, including AFP, in remarks made public Friday.

"Their main task is to penetrate as deeply as possible into our territory, to withdraw, regroup and advance again -- then attack targets deep inside to paralyse our logistics and troop rotations," he explained.

He said the tactics allow Russian forces to push forward with relatively few troops -- though means they do not gain full territorial control of an area.

Russian troops managed to pierce Ukraine's defence in August near the eastern coal mining town of Dobropillia before Kyiv pushed them back.

Syrsky said the overall situation on the front -- which stretched 1,250 kilometres (775 miles) -- was difficult, with Russia advancing in key sections.

Russia's defence ministry on Friday claimed the capture of Yunakivka, a village in the northeastern Sumy region.

Syrsky also said Ukraine was changing some of its own air defence techniques to counter increasing Russian aerial strikes on its cities.

That included the increased deployment of helicopters and light aircraft as well as the development of new drone interceptors.