Ukrainian troops may need to retreat from the frontline city of Lysychansk to avoid encirclement after Russian forces captured two settlements to its south, regional governor Serhiy Gaidai said on Thursday.
Lysychansk and nearby Sievierodonetsk have become the focus of Russia's offensive in the eastern Donbas region and the battle there is approaching a "fierce climax," a Ukrainian presidential adviser said on Wednesday.
The general staff of Ukraine's armed forces on Thursday confirmed the loss of Rai-Oleksandrivka and Loskutivka, which lie around 5 km (3 miles) from Lysychansk, and said Russian troops were trying to surround Ukrainian forces there.
It did not say how it would respond, but Governor Gaidai said Ukrainian troops might have to draw back.
"In order to avoid encirclement, our command could order that the troops retreat to new positions," he said on national television.
"All of Lysychansk is within reach of their fire. It is very dangerous in the city."
He said Lysychansk could still be reached by road, allowing civilian evacuations to continue. Russia's TASS news agency had earlier cited Russian-backed separatists as saying the city was surrounded and cut off from supplies.
The war has entered a brutal attritional phase in recent weeks, with Russian forces concentrating their overwhelming artillery firepower on this pocket of the Donbas, which Moscow claims on behalf of separatists.
Britain's defence ministry said on Thursday Russia's recent advances were putting the Ukrainian forces holding out in Lysychansk-Sieverodonetsk under increasing pressure.
"However, its (Russia's) efforts to achieve a deeper encirclement to take western Donetsk Oblast (region) remain stalled," the ministry said in a daily update on Twitter.