The fall of Siversk comes with Ukraine under U.S. pressure to quickly negotiate peace in the nearly four-year-old war triggered by Russia’s invasion, and puts Russian forces closer to the hub of Sloviansk, some 30 km (20 miles) to the west.

Sloviansk is a northern anchor of the so-called “fortress belt” of cities in Ukraine’s heavily industrialised Donbas region, which Russia has demanded Kyiv cede before it ends its war.

“The invaders were able to advance due to a significant numerical advantage and constant pressure from small assault groups in difficult weather conditions,” Ukraine’s General Staff said in a statement.

It said it had withdrawn soldiers to preserve lives and resources, adding that they had, however, inflicted heavy losses on the enemy.

Russia said earlier this month it had captured Siversk, whose pre-war population was around 10,000, a claim Kyiv had denied.

On Tuesday, Ukraine’s military said its forces were keeping up pressure on the town and would continue trying to sever logistics for Russian troops inside.

Ukrainian open-source group Deep State showed Russia had made gradual gains around Siversk this month, part of its grinding advances along the sprawling front line.

Fighting is still raging around the former eastern logistics hub of Pokrovsk, where Ukrainian forces said this month they had fended off an unusually large Russian mechanised attack.