Russia has likely ordered its troops to withdraw from the Kharkiv region amid major Ukrainian gains, according to the British Ministry of Defence (MoD).
It said Kyiv had recaptured territory double the size of Greater London since last Wednesday.
"The rapid Ukrainian successes have significant implications for Russia’s overall operational design," the ministry wrote. "The majority of the force in Ukraine is highly likely being forced to prioritise emergency defensive actions."
"Ukrainian forces have penetrated Russian lines to a depth of up to 70 kilometres in some places and captured over 3,000 square kilometres of territory in the past five days since September 6 - more territory than Russian forces have captured in all their operations since April," said the Institute for the Study of War in its latest assessment.
Russian forces, the Institute said, "are not conducting a controlled withdrawal and are hurriedly fleeing southeastern Kharkiv Oblast to escape encirclement around Izyum".
Video clips on social media show scores of Russian military vehicles including tanks and armoured personnel carriers, abandoned by the side of the road as their crews seemingly fled ahead of the Ukrainian surge.
Other videos show grateful civilian residents, liberated after six months under Russian occupation, crying and hugging Ukrainian soldiers who advanced so far, so fast, they were able to reach the town of Hoptivka, just 2km from the Russian border.
So what happens next? The Institute for the Study of War predicts that Ukrainian forces "will likely capture" Izyum within the next 48 hours, which they say would mark the most significant Ukrainian victory since winning the Battle of Kyiv in March.
A Russian announcement about troops
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