Ukrainian forces appear to have seized two villages on a surprise offensive south of Kharkiv that has broken Russia's frontlines.
Fighting near the town of Balakliya was first reported on Tuesday morning in what quickly emerged into a Ukrainian attack.
Ukraine's army and government did not announce the assault and have refused to comment on its progress or objectives.
But unofficial Russian and Ukrainian Telegram channels on Wednesday morning confirmed significant overnight advances including the capture of two settlements.
The reports say Ukrainian troops have surrounded the town of Balakliya and likely seized the settlement of Volokhiv Yar, 10 miles northwest.
Ukrainian soldiers in recently captured Russian positions near occupied Balakliia, Kharkiv Oblast.????
A captured Russian MT-12 "Rapira" 100mm towed anti-tank gun can be seen in the background.pic.twitter.com/pnnuWfFp1k
Further fighting was reported near the town of Shevchenko, another dozen miles along the same road.
If confirmed, the advances would give the Ukrainians control of a road threatening a bottleneck in Russian supply lines to its forces fighting further south.
Konrad Muzyka of Rochan Consulting, which publishes daily analysis of the war, said the Ukrainian information blackout means it is too early to assess the scale of the operation, its objectives or how dangerous it might prove to the Russians.
"To that extent we have to rely on what Russians are saying that makes me feel quite uncomfortable," said Konrad Muzyka of Rochan Consulting.
"But there are images posted by Ukrainian soldiers and geolocated that confirm their presence across the river in Kharkiv region that seems to confirm an offensive happened and they had some successes."
"In general we are probably looking at a stalemate - however we are probably seeing the first evidence that the balance is slowly shifting to Ukraine's favour," he added.
Visual confirmation that Volokhiv Yar has been retaken by Ukraine Armed Forces. Balakliya cut off from Kupyansk. pic.twitter.com/oKlh0OuK0b
Pro-Ukrainian channels on Wednesday morning posted a video of infantrymen in an area of ruined buildings and woodland posted by Ukrainian with the caption "Verbivka is ours again." Verbivka is a hamlet just outside Balakliya.
Another showed a group of men in light vehicles wearing blue armbands worn by Ukrainian troops on the roadside.
The Telegraph was able to geolocate the video to the central intersection at Volokhiv Yar.
The direction of shadows suggests it was filmed in the late evening, which would mean the Ukrainians were in the village before sunset on Tuesday.
Russian Telegram channels run by mercenaries, soldiers and war bloggers reported significant Ukrainian gains.
"Volokhiv Yar, Semyonovka are under the Nazis. They are moving to Shevchenko," said one post widely shared by pro-Russian channels, including one controlled by the Wagner private military company.
Russian propaganda refers to Ukrainians as Nazis. The loss of the Volokhiv Yar was confirmed by Rybar, another widely read war blogger.
???Advancing Ukrainian troops captured a Russian Lieutenant colonel near Balakliia in #Kharkiv Oblast. #UkraineRussiaWar pic.twitter.com/cy3OV25SIi
Alexander Kots, a war correspondent for Komsomolskaya Pravda, a Russian tabloid, wrote on his personal Telegram channel that the Ukrainians were likely seeking to cut off the main Russian command centre in this part of the front.
"Taking Balakleya at all costs is not the Ukrainians' task" he said, citing assaults north of the town.
"For them it is important to firstly control the road from Volokhiv Yar to Balaklieya (their artillery already controls it), the latter is effectively surrounded. From Volkhov Yar there is a direct road to Izyum and also to Shevchenko, through which the road reaches Kupiansk.
"If the enemy moves through Shevchenko to Kupiansk, the Izyum grouping will in fact be cut off from the [Russian] mainland."
Izyum, a key junction on the border of the Kharkiv and Donbas regions, was captured by the Russians in March ahead of its summer offensive in Donbas.
Losing it would be a major blow to Russia and help the Ukrainians ease pressure on towns like Sloviansk on the northern flank of the Donbas salient.
Kupiansk lies to the north of Izyum and straddles the main road linking it to Russia.
Mr Kots suggested in his post that the Russians near Izyum should consider evacuating east to avoid encirclement, but noted such an operation would take time.
He added that steps should be taken to reform a system of command in which "the leadership does not like bad news and subordinates do not like to anger their superiors."
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