Russian forces in the Donetsk region town of Lyman are on the brink of encirclement following Ukrainian gains to their north and east, pro-Russian military bloggers have said.
The claims could not be immediately verified, but the reported Ukrainian gains appeared to match images and videos released on social media by Ukrainian soldiers.
The fall of the town would be another military set-back for Russia and position the Ukrainians to push into Luhansk region, reversing the hard-won gains of Russia's summer offensive in Donbas.
It could be even more damaging politically, demonstrating that Russia has failed to stabilise its defensive line following the loss of Izyum and Kupyansk earlier this month.
'The city is in an extremely vulnerable situation'
"Ukrainian soldiers are releasing images of them raising a flag over the village council building in Novosyolovka. Thus from the north between Lyman and enemy forward elements of the remains only one settlement, Drobyshevo, and in all just a few kilometers of land.
"The city is in an extremely vulnerable situation," a Russian military telegram channel called Military Observer reported on Wednesday afternoon.
Rybar, another authoritative Russian military blogger, said Russian forces had repelled Ukrainian attacks in a number of areas to the north, west, and east of Lyman, but published a map showing Russian control confined to a narrowing salient.
The Telegraph could not immediately verify the map. If genuine, it would suggest the Ukrainians are close to closing a pocket around the city.
Russia forces captured Lyman, in northern Donetsk region, in May. It was intended to be used as a launch pad for an assault on Ukraine's stronghold towns of Slovyansk and Kramatorsk.
The garrison there was left vulnerable after a sudden Ukrainian offensive in early September led to the collapse of the Russian army group in Kharkiv region.
Ukrainian forces have this week attempted to expand a bridgehead on the eastern bank of the Oskil river, where Russia had attempted to regroup after that retreat.
Retreat might be preferable to seeing Russian forces surrounded in a pocket, however.
Russia's ability to surround and destroy Ukrainian forces in "cauldrons" caused major blows to Ukrainian morale during the 2014 invasion, and led some Western experts to argue Ukraine could never win a conventional war in the run up to this year's invasion.
The sight of Ukraine doing the same thing to Russian troops would be another critical blow to the credibility of the Kremlin's high command and further vindicate those in the West who believe Ukraine can win the war.
It is not clear how many Russian troops are inside Lyman, but reports on social media channels suggest the presence of a mixture of regular Russian troops and fighters from the so-called Luhansk and Donetsk People's Republics, the puppet statelets Russia set up in occupied territory in 2014.
The "People's Militia" of the Donetsk People's Republic said on Telegram that the main roads in and out of Lyman remained under Russian control.