Ukrainian troops have raised the nation's flag over the main railway city that has supplied Russian forces in northeastern Ukraine, as a collapse in Russia's frontline threatened to turn into a rout.
A Reuters journalist inside a vast area recaptured in recent days by the advancing Ukrainian forces saw Ukrainian police patrolling towns and boxes of ammunition lying in heaps at positions abandoned by fleeing Russian soldiers.
With Ukrainians now having reached the city of Kupiansk, where rail lines linking Russia to eastern Ukraine converge, the advance had penetrated all the way to Moscow's main logistics route, potentially trapping thousands of Russian troops.
Natalia Popova, adviser to the head of the Kharkiv regional council, shared photos on Facebook on Saturday of troops holding up a Ukrainian flag in front of Kupiansk city hall.
A Russian flag lay at their feet. "Kupiansk is Ukraine. Glory to the armed forces of Ukraine," she wrote.
Ukraine's security service confirmed Kyiv had forces inside Kupiansk.
In Hrakove, one of dozens of recaptured villages, Reuters saw burnt-out vehicles bearing the "Z" symbol of Russia's invasion, and piles of rubbish and ammunition in positions the Russians had abandoned in evident haste.
"Hello everyone, we are from Russia," was spraypainted on a wall.
Three bodies lay in white body bags in a yard.
The regional chief of police, Volodymyr Tymoshenko, said Ukrainian police had moved in the previous day, and had checked the identities of local residents who had lived under Russian occupation since the invasion's second day.
"The first function is to provide help that they need. The next job is to document the crimes committed by Russian invaders on the territories which they temporarily occupied."
The capture of at least part of Kupiansk, if confirmed, potentially leaves thousands of Russian soldiers trapped at the frontline and cut off from supplies, including in Izium, Russia's main stronghold and logistics hub in the northeast.
Reuters could not independently verify the situation in either Kupiansk or Izium. Moscow has acknowledged that its frontline has buckled in Kharkiv but has said it is rushing extra troops to reinforce the area. Russian-installed regional officials have called for civilians to evacuate both cities.
Britain's Ministry of Defence in an intelligence update said the Russian force around Izium is "likely increasingly isolated".
"Ukrainian units are now threatening the town of Kupiansk; its capture would be a significant blow to Russia because it sits on supply routes to the Donbas front line."
Mark Hertling, a retired four-star general and former commander of US ground forces in Europe, tweeted: "Make no mistake, (Ukraine) is executing a brilliant manoeuvre focused on terrain objectives to 'bag' Russians. But the Russians are helping them -- by doing very little to counter."
In an overnight video address, President Volodymyr Zelenskiy said at least 30 settlements had been liberated in Kharkiv region during the advance of recent days.
"Our army, intelligence units and the security services are carrying out active engagements in several operational areas. They are doing so successfully," he said.