Moscow has abandoned its main bastion in north-eastern Ukraine in a sudden collapse of one of the war's principal frontlines after surging Ukrainian forces made a rapid advance.
Ukrainian forces pushed up to 50 kilometres past Russian lines this week and recaptured dozens of towns in a dramatic counteroffensive.
The swift fall of three key cities — Izium, Kupiansk and Balaklyia — in Kharkiv province was Moscow's worst defeat since its troops were forced back from the capital Kyiv in March.
The counteroffensive comes as the state agency in charge of the Russian-held Zaporizhzhia Nuclear Power Plant in Ukraine said operations at the plant had been fully stopped on Sunday.
The agency said it had disconnected the sixth unit of the plant from the power grid.
Speaking about the success of the counteroffensive, Ukraine's Foreign Minister Dmytro Kuleba said the supply of weapons from the west was paying off.
"We showed that we can defeat the Russian army," he said.
"I repeat again the more weapons we receive the faster we win, the faster this war will end and we will be able to concentrate on other goals."
This could prove a decisive turning point in the 6-month-old war, with thousands of Russian soldiers abandoning ammunition stockpiles and equipment as they fled.
Russian forces used Izium as the logistics base for one of their main campaigns — a months-long assault from the north on the adjacent Donbas region comprised of Donetsk and Luhansk.
Russia's state-run TASS news agency quoted the Defence Ministry as saying it had ordered troops to leave the vicinity and reinforce operations elsewhere in neighbouring Donetsk.
The head of Russia's administration in Kharkiv told residents to evacuate the province and flee to Russia to "save lives", TASS reported.
Witnesses described traffic jams of cars with people leaving Russian-held territory.
Advances cut Russian supply lines
The Russian withdrawal was heralded by Ukrainian leaders.
President Volodymyr Zelenskyy said in a video address on Saturday that Ukraine's armed forces had liberated around 2,000 square kilometres of territory since a counter-offensive against Russia started earlier this month.
In a video released by his office, Mr Zelenskyy said it was "a good decision for [the Russians] to run by showing their backs".
Ukrainian officials had earlier stopped short of confirming they had recaptured Izium, but Andriy Yermak, Mr Zelenskyy's chief of staff, posted a photo of troops on its outskirts and tweeted an emoji of grapes. The city's name means "raisin".
The Russian army was claiming the title of "fastest army in the world … keep running!" Yermak later wrote on Twitter.
The Russian withdrawal announcement came hours after Ukrainian troops captured the city of Kupiansk farther north, the sole railway hub supplying Russia's entire front line across north-eastern Ukraine.
Ukrainian officials posted photos early on Saturday of their troops raising the country's blue-and-yellow flag in front of Kupiansk's city hall.
That left thousands of Russian troops abruptly cut off from supplies along a front that has seen some of the most intense battles of the war.
There were signs of trouble for Russia elsewhere along its remaining positions at the eastern front, with pro-Russian officials acknowledging difficulties at other locations.
Ukrainian armed forces are continuing to advance in different areas along the front, Mr Zelenskyy said.
'Russia is retreating'
A witness in Valuyki, a town in Russia's Belgorod region near the border with Ukraine, said she saw families from Kupiansk eating and sleeping in their cars along roads.
"I was at the market today and saw a lot of people from Kupiansk. They say half of the city was taken by the Ukrainian army and Russia is retreating … the fighting is getting closer," the witness said.
Belgorod Governor Vyacheslav Gladkov said officials were giving food and medical aid to people queuing at a crossing into Russia.
Senator Andrey Turchak, from the pro-Kremlin United Russia party, reported more than 400 vehicles at the frontier.
Russian rocket fire hit Kharkiv city on Saturday evening, killing at least one person and damaging several homes, part of a surge in shelling since Kyiv's counteroffensive, Ukrainian officials said.
Neither battlefield accounts could be independently verified.
There were signs that Ukraine could capitalise on the disarray with assaults along other areas of the eastern front.
Denis Pushilin, head of the Russian-installed separatist administration in Donetsk province, said the situation in Liman, east of Izium, "remains quite difficult — as in a series of settlements in the north of the republic".
A bit further east, Ukrainian officials hinted at a possible attempt to recapture Lysychansk, which Moscow seized in July after weeks of fighting in one of the war's bloodiest battles.
Ukrainian regional Governor Serhiy Gaidai was quoted in media as saying Ukrainian troops had been spotted on the city's outskirts.
The city's name means "fox", and after his tweet of grapes, Andriy Yermak, Mr Zelenskyy's chief of staff, tweeted a fox emoji.
ABC/wires
