Russian troops on Monday captured the east Ukraine town of Kreminna, local authorities said, as Kyiv's armed forces launched salvoes on Russian forces in the nearby settlement of Rubizhne.
"There was a major attack in the night" from Sunday to Monday in Kreminna, the Lugansk regional governor Sergiy Gaiday said in a statement on social media.
"The Russian army has already entered there, with a huge amount of military hardware... Our defenders have retreated to new positions," he added.
The capture of the city of Kreminna may have heralded the start of a widely anticipated major Russian offensive in Ukraine's eastern Donbas region as fierce fighting raged in neighbouring Rubizhne.
Moscow has decided to refocus its efforts on eastern Ukraine after withdrawing troops from the region around the capital Kyiv, creating a new epicentre in the conflict.
The Donets river skirts Kreminna and snakes through nearby Rubizhne, Severodonetsk and Lysychansk, which have withstood heavy shelling from both sides for several days on the frontline.
"We can now confirm that Russian troops have begun the battle for the Donbas, which they have been preparing for a long time," Ukrainian President Volodymyr Zelensky later said on Monday evening.