Ukrainian soldiers have broken through Russia's frontline along the Oskil river, exposing an important supply route and swathes of territory the Kremlin has promised to defend.
Fighting has focused on Kupiansk since Russian forces fled a surprise Ukrainian offensive around Kharkiv in northeast Ukraine last week but now military bloggers from both sides have said Russia's defence has crumbled once again.
"Kupiansk has surrendered. The enemy is on the left bank of the Oskil," the pro-Russian military blogger Vladlen Tatarsky told his 450,000 subscribers last night.
Western commentators confirmed the development. James Rushton, a British military analyst based in Ukraine, said he didn't think Russian forces would be able to hold their defensive line along the Oskil River for much longer.
"The Ukrainians have already crossed it," he said. "Considering they now control all of Kupiansk, half of the city is on the eastern bank, any defensive line the Russians might try to form along the river is already very vulnerable."
Russian forces fled across the Oskil River, which runs north-south from central Russia into Ukraine, after a surprise Ukrainian offensive recaptured about 4000sq km of territory.
Kupiansk and Izyum are the largest and most valuable towns captured by the Ukrainian military. Both were heavily fought over in the opening weeks of the war and the Russian army had used them as logistics bases for supplying its forces in Donbas.
The British Ministry of Defence said a Ukrainian breach of the Russian frontline along the Oskil River would humiliate the Kremlin, which has made capturing and then defending Luhansk a priority and threaten its final logistics hub to Donbas from the Russian city of Belgorod.
"This line sits along the border of Luhansk Oblast, part of the Donbas, which Russia aims to 'liberate' as one of its immediate war aims," it said. "Any substantial loss of territory in Luhansk will unambiguously undermine Russia's strategy."
It added that "Russia will likely attempt a stubborn defence of this area, but it is unclear whether Russia's front line forces have sufficient reserves or adequate morale to withstand another concerted Ukrainian assault."
The Oskil River meanders south through fertile farmland in Ukraine before joining the Seversky Donets River which flows into Donbas. It lies 120km east of Kharkiv and 175km to the northwest of the city of Luhansk which pro-Russia rebels have controlled since an uprising in 2014 against the central government in Kyiv.
The US-based Institute for the Study of War also confirmed Ukrainian forces had broken through Russia's northeastern frontline.
"Russian forces are likely too weak to prevent further Ukrainian advances along the entire Oskil River," it said.
A video reportedly shot from a Ukrainian tank and uploaded onto Twitter was geolocated to Kupiansk on the east bank of the Oskil River.
It showed the smashed-up and destroyed city. On the left of the video footage lie the wrecks of two smouldering and seemingly abandoned Russian tanks, one painted with the letter Z, which has become a Russian pro-war symbol.
Another video of a Ukrainian soldier celebrating the destruction of a Russian armoured personnel carrier has also been geolocated to the east bank of the Oskil River.
"All will soon be Ukraine," the soldier says in the video. Russian forces have taken huge losses since the Kremlin ordered the invasion of Ukraine on February 24.
The Pentagon has estimated the Russian army has sustained at least 75,000 dead or injured, far greater than the Soviet Union's losses in a decade of fighting in Afghanistan in the 1980s.