Vladimir Putin's army has suffered a devastating string of defeats putting Russia on the back foot as Ukraine reclaims vast swathes of territory in the ongoing war.
Moscow abandoned its main bastion in northeastern Ukraine on Saturday (local time) in a sudden collapse of one of the war's principal front lines.
The sudden advancement of Ukrainian forces has surprised many with some international observers rushing to declare a major shift in the conflict.
"Just in case you are still not informed. Ukraine has launched the greatest counteroffensive since World War II," exclaimed John Spencer from the Modern War Institute in the United States.
"Ukraine has regained/liberated over 1,000 km of land and cities. Ukraine is winning. Ukraine is defeating Russia," he tweeted Sunday.
The swift fall of Izium in Kharkiv province – which has been confirmed by reporters on the ground – was Moscow's worst defeat since its troops were forced back from the capital Kyiv in March.
Ukraine hailed it as a turning point in the 6-month-old war, with thousands of Russian soldiers leaving behind 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.
The state-run TASS news agency quoted Russia's defence ministry as saying it had ordered troops to leave the vicinity and reinforce operations elsewhere in Donetsk, Reuters reported.
Despite the Kremlin's spin, Russian propagandist channels on Telegram have reportedly labelled the retreat "a catastrophe", even calling Putin an "idiot".
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.
If the reported gains are held, it would be a serious blow for Russia, which Western intelligence services say has suffered huge casualties.
Fears over Putin's next move
Ukraine has enjoyed immense backing from western countries and is keen to show the weapons it has been supplied with can help turn the war in its favour. But some are concerned over how the Russian leader will respond to his latest setback.
Journalist and analyst Oliver Alexander expressed concern over the potential of a brutal retaliation on the part of Putin.
"This is starting to look more and more like a complete collapse of Russian forces in the Kharkiv region. Every hour there are reports of Russian troops leaving a new town," he tweeted Sunday.
"While good, I am starting to get a bit worried as to what Russia will do in an attempt to stop it."
He wasn't the only one.
"In light of the unfolding collapse of Russia’s army in Eastern Ukraine, what is now preventing Putin from using tactical nuclear weapons?" wondered Gordon G. Chang, author of the Coming Collapse of China.
'We will win': Ukraine Foreign Minister
There is pressure on Kyiv to demonstrate progress before winter sets in, amid threats by Russian President Vladimir Putin to halt all energy shipments to Europe if Brussels goes ahead with a proposal to cap the price of Russian oil exports.
Ukraine's Foreign Minister Dmytro Kuleba said in Kyiv that Ukrainian forces had demonstrated they were capable of defeating the Russian army with the weapons given to them.
"And so I reiterate: the more weapons we receive, the faster we will win, and the faster this war will end," he said.
In his nightly video address on Saturday, President Volodymyr Zelenskiy said Ukraine's armed forces had recovered around 2,000 square kilometres of territory since its counter-offensive was launched earlier this month.
"The Russian army is claiming the title of fastest army in the world ... keep running!" Andriy Yermak, Zelenskiy's chief of staff, 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 northeastern 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.
Igor Girkin, a former commander of pro-Russian forces in eastern Ukraine, called the Russian pullback "a major defeat" in remarks on Telegram.
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