Russia has made sweeping advances in recent days that threaten to outweigh the gains made by Ukraine in its cross-border attack into the Kursk region.
Russian forces are just a few kilometres from the Ukrainian city of Pokrovsk, a crucial logistics hub used by the Ukrainian military.
Home to a key railway station and major roads, Pokrovsk is an essential supply and reinforcement point for Ukraine’s troops on the eastern front line.
Critics in Kyiv fear that the country's military has made a serious miscalculation.
By
sending troops into Kursk instead of reinforcing the eastern frontline, the military has left Pokrovsk and other important Ukrainian towns exposed, these critics say.
Russian President Vladimir Putin launched a full-scale invasion of Ukraine in February 2022.
On a visit to the front line, Ukraine's armed forces chief Gen Oleksandr Syrskyi said Russia was throwing “everything that can move” into its assault.
“The situation is extremely difficult,” Ukrainian President Volodymyr Zelensky conceded on Wednesday.
“If we lose Pokrovsk,” military expert Mykhaylo Zhyrokhov warned, “the entire front line will crumble."
Pokrovsk is next to another town, Myrnohrad. Together, the two settlements had a pre-war population of over 100,000, most of whom have now fled. They are the last major cities in that part of the Donetsk region that remain under Ukrainian control.
The battle for Pokrovsk is really a continuation of the
battle for Avdiivka, which Ukraine lost in February
after months of bloody fighting.
Avdiivka, which is about 40km (25 miles) south-east of Pokrovsk, was seen as a fortress that protected the settlements and roads to its west - helping to bolster Ukraine’s presence along the entire frontline.
When it finally fell, Avdiivka was left in ruins. It was a serious loss for Ukraine.
It meant that Russia could move its focus to Pokrovsk and the key hilltop town of Chasiv Yar, which overlooks some of the important cities in Donetsk still under Ukrainian control. Intense fighting there on Saturday left five people dead.