(Bloomberg) -- Thousands of Russian troops retreated in the face of a lightning Ukrainian offensive in the Kharkiv region that threatens to derail the Kremlin’s bid to cement control of Ukraine’s east.  

A local Moscow-backed official and pro-Russian military bloggers said Saturday that Kremlin forces had pulled out of Izyum, a staging post for the campaign in Donbas, to avoid being encircled. 

Russia’s defense ministry confirmed the pullout from Izyum and other areas in a statement from Moscow, describing the move as necessary to bolster forces in the eastern Donetsk region. Ukrainian President Volodymyr Zelenskiy said the Russian forces were making “a good decision” to leave the area, according to a video released by his office Saturday. 

The news came hours after Ukrainian officials announced the seizure of Kupyansk, a logistical and transit hub for Russian troops fighting in the east, and the recapture of other occupied territory in the northern Kharkiv region earlier this week.

Ukraine’s defense ministry also tweeted a photo it said was of the liberation of Balakliya, to the southwest of Kupyansk, with troops shown raising a Ukrainian flag in the town center. 

The advances come after Kyiv received increasing supplies of weapons from the US and its allies over the past few months, including powerful HIMARS rocket launchers, giving Ukraine’s backers confidence that it can push back against the Russian invasion, now in its seventh month. 

Read more: Ukraine Army’s Breakthrough in North Threatens Russian Grip  

Daniil Bezsonov, first deputy minister of information for the Moscow-backed Donetsk People’s Republic in Donbas, said the Russian military had abandoned Izyum and some other localities in Kharkiv.

“Of course, this is the result of high command mistakes,” he said on his Telegram channel. 

Igor Girkin, a former Russian intelligence colonel who became a commander of the pro-Russian separatist forces in Donbas in 2014, also confirmed the retreat from Izyum.

“A hasty retreat will inevitably lead to heavy losses by the retreating troops in equipment and munitions and -- most sensitive of all -- in people,” Girkin said in a Telegram post.  

The Russian defense ministry said on its Telegram channel that the decision had been taken to reallocate forces from Balakliya and Izyum to Donetsk over a three-day period to “achieve the stated goals of liberating Donbas.” 

Russian military correspondents have voiced increasing concern about the reverses, with Ukrainian forces advancing dozens of kilometers into Russia-controlled territory in Kharkiv over the past several days.

“The situation for the Russian armed forces in the battles near Kharkiv is the hardest,” Yevgeny Poddubny, a war correspondent of State Russian TV channel VGTRK, said on his Telegram channel on Saturday.

Reinforcements Coming

The arrival of reinforcements “slowed down but didn’t stop” the Ukrainian counter-offensive, he said.  

After pulling back following a failed bid to capture the Ukrainian capital Kyiv, Kremlin forces since April switched their attention to Donbas, the eastern provinces of Donetsk and Luhansk. 

But despite overrunning Luhansk, Russia has struggled to advance further west. And since the end of August Ukraine started a new push to dislodge Moscow’s forces from both the southern Kherson region and Kharkiv in the north, targeting Russian logistics capability and forcing resources to be divided.

Zelenskiy said on Thursday night that Kyiv’s troops had recaptured about 1,000 square kilometers in Kharkiv. The Institute for the Study of War, a US-based think tank, now estimates the area at about 2,500 square kilometers. 

In Moscow, there was no outward sign of Kremlin concern about the sudden battlefield setbacks. 

President Vladimir Putin continued with his planned public schedule, presiding over the opening of a new Ferris wheel and martial-arts gym in Moscow. State television showed brief videos of what it said were reinforcements headed for the Kharkiv area.

At a concert in honor of the Day of the City of Moscow, Putin said the Russian soldiers who died during what the Kremlin calls a special military operation in Ukraine gave their lives for the country, TASS reported. 

(Updates with comment from Zelenskiy in third paragraph.)

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