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 British defense ministry said in an intelligence briefing Sunday.
Russian forces have likely “withdrawn units from the area,” but fighting continues around the strategically important cities of Kupiansk and Izyum, it said in its daily update on the war in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 Vladimir Putin, said in a Telegram post Sunday that "mistakes were made" by the Russian defense ministry 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, Ukrainian President Volodymyr Zelenskyy 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 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 Sunday, Ukraine's army chief said his forces had recaptured 3,000 square kilometers (about 1,860 square miles) of territory from Russia since the counteroffensive was launched in the beginning of this month. NBC News could not verify this claim.
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 counteroffensive 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 counteroffensive 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 power plant in Ukraine has been shut down.
Energoatom, the state-owned company in charge of the plant, said Sunday that work at the facility had been “completely stopped” after it disconnected the No. 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 accused each other of damaging power lines at the Zaporizhzhia site with rocket and artillery fire, risking a potential nuclear disaster.
Ukrainian media reported widespread power outages Sunday night in Kharkiv, Poltava, Donetsk, Zaporizhzhia, Sumy and other regions. Officials in several regions said Russian forces caused the outages by attacking infrastructure, knocking out electricity and water, with explosions.
Kharkiv Mayor Igor Terekhov said the power outage was “revenge by the Russian aggressor for the successes of our army at the front, in particular, in the Kharkiv region.”
Local officials said they were trying to repair the damage, and none of the outages were believed to be related to the shutdown of the reactors at the Zaporizhzhia plant.