Ukraine's armed forces have recaptured large swaths of territory and are making "significant gains" against Russia's occupation of the northwest region of Kharkiv, the U.K.'s defense ministry said in an intelligence briefing Sunday.
Russian forces have likely "withdrawn units from the area," but that fighting continues around the strategically important cities of Kupiansk and Izyum, it said in it daily update on the war in Ukraine, posted to Twitter.
Ukraine's government claims Russia's retreat from Kharkiv is a major turning point in the six-month-old conflict, as thousands of Russian soldiers abandoned their weapons and ammunition stockpiles to flee the Ukrainian advance, they said.
Some Western experts have described the withdrawal as the worst defeat for Russian forces since they were forced back from the capital Kyiv in March.
Ramzan Kadyrov, the leader of Chechnya and a loyal ally to Russian President Putin, said in a Telegram post on Sunday that "mistakes were made" by the Russian Ministry of Defense in its strategy. "If changes are not made today or tomorrow in the strategy of conducting a special operation, I will have to contact the leadership of the Ministry of Defense," he added.
On his Telegram channel Sunday, Ukraine's president Volodymyr Zelenskiyy said the Ukrainian flag had "returned to Chkalovske, Kharkiv region."
"It will be like that everywhere. We will expel the occupiers from every Ukrainian town and village," he added.
Russia's Defense Ministry said in a Telegram post Saturday that "a decision was made to regroup" some of its troops from the Balakliya and Izyum areas - Izyum had been a major base for Moscow's troops - and transfer them to Ukraine's eastern Donetsk region.
The move was made "in order to achieve the stated goals of the special military operation to liberate Donbas,'" the ministry said, referring to the industrial heartland in Ukraine's east that became the focal point of the Kremlin's war after it was forced to give up on its assault on the capital, Kyiv.
NBC News could not independently verify the claims of either side.
But a former commander of pro-Russian forces in eastern Ukraine, Igor Girkin called the Russian pullback "a major defeat" in comments on his Telegram channel.
During a news conference on Sunday, Ukraine's army chief said his forces had recaptured 3,000 km (about 1,860 miles) of territory from Russia since the counter-offensive was launched in the beginning of September. NBC News could not verify this claim.
Russian President Vladimir Putin promised earlier this week to continue Moscow's military efforts in Ukraine, saying that his country was gaining rather than losing from the conflict.
Events on the battlefield appeared to paint a bleak picture for the Kremlin, however.
Ukraine initially launched a counter-offensive in the country's south late last month after weeks of public buildup and preparation, as it aimed to push toward Kherson, a key city near the southern coast.
Then this week, after Russia redeployed large numbers of its own forces to the south to combat that effort, reports began to emerge of Kyiv's forces launching another counter-offensive farther north - a move that appeared to catch both the broader world and Moscow's military off guard.
Elsewhere in Ukraine, the last operational reactor at the Russian-held Zaporizhzhia nuclear plant in Ukraine has been shut down.
Energoatom, the state-owned company in charge of the plant, said on Sunday that work at the facility was "completely stopped" after it disconnected the Number 6 power unit from the grid at 3:41 a.m. (8:41 p.m. ET).
"A decision was made to shut down power unit No 6 and transfer it to the safest state - cold shutdown," it said on Telegram.
The Zaporizhzhia facility, Europe's largest nuclear power plant, was cut off from the grid last week after all its power lines were disconnected as a result of fighting in the area. It ran on "island mode" for several days, officials said, generating electricity for crucial cooling systems from its only remaining operational reactor.
Energoatom said it restored a communications line to the power system, allowing the plant to be powered by Ukraine's energy grid long enough to initiate the shutdown.
The company said the shutdown took place because the risk of further damage to the power lines "remains high," which would disconnect the plant completely from the grid again.
Russia and Ukraine accuse each other of damaging power lines at the Zaporizhzhia site with rocket and artillery fire, risking a potential nuclear disaster.
This article was originally published on NBCNews.com