The lightning advance has set the scene for what appears to be a looming confrontation over the two key areas, seen as strategically important by both sides. Russia is currently using Kupiansk as a rail hub to resupply its forces and Izium as a launching pad for attacks southwards into the Donetsk region.

“The helicopters are dropping in at landing points all along the line of contact… The reinforcements will help to stop any further advance of the Kyiv regime,” Poddubny said.

In a later report he said that Kupiansk had come under artillery fire from Ukrainian armed forces and that a bridge across the Oskil River in the city had been seriously damaged.

“Our reserves are arriving at the front line,” Poddubny said. “The defense of Kupiansk continues.”

Meanwhile, Ukrainian President Volodymyr Zelensky said Friday his troops had “liberated and taken control of more than 30 settlements in the Kharkiv region.”

“Measures to check and secure the territory in some villages of the region are ongoing, we are gradually taking control of new settlements — we are returning the Ukrainian flag and protection for all our people everywhere,” Zelensky said in his daily address.

Scenes of destruction

The advancing Ukrainian troops have reported scenes of destruction as they retake areas from Russian control.

The head of the Kharkiv region military administration, Oleh Syniehubov, said he had toured recently liberated areas where the Russians had “left broken infrastructure and houses, many trophies and piles of garbage,” and posted photographs of the scenes on Telegram.

“People, without exaggeration, met our soldiers with tears in their eyes,” Syniehubov said.

“Residents, of course, need help. Most villages do not have electricity or gas. In the near future, we will restore them and bring in humanitarian aid.”

He said the head of one liberated village had been detained on suspicion of collaborating with the occupying Russian forces, and warned any other collaborators that “no traitor will escape responsibility.”

Falling morale

Ukraine says some Russian troops are now deserting due to the “significant losses” they have suffered.

The General Staff of the Ukrainian Armed Forces said Friday that at least 15 desertions had taken place.

Soldiers were abandoning their uniforms and wearing civilian clothes in an effort to blend in while attempting to return to Russian territory, it said.

Video and satellite images that have been geolocated by CNN show those advances have involved sustained attacks on command posts, ammunition stores, and fuel reserves far behind the front lines.

A senior US official said Ukrainian forces had achieved some success in attacking Russian supply lines, with the intention of cutting off and isolating Russian troops west of the Dnipro River.