Ukrainian forces vowed to regroup and fight from "higher ground" after the military acknowledged the fall to Russian forces of the key eastern city of Syevyerodonetsk following a long, brutal battle.
Kyrylo Budanov, head of Ukraine's military intelligence agency, told Reuters late on June 25 that the country’s forces would continue their eastern defense from embattled Lysychansk after the loss of Syevyerodonetsk, located just across the Siverskiy Donets River.
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"The activities happening in the area of Syevyerodonetsk are a tactical regrouping of our troops. This is a withdrawal to advantageous positions to obtain a tactical advantage," Budanov said.
"Russia is using the tactic...it used in Mariupol: wiping the city from the face of the Earth. Given the conditions, holding the defense in the ruins and open fields is no longer possible. So the Ukrainian forces are leaving for higher ground to continue the defense operations," he said.
In a late-night address, Ukrainian President Volodymyr Zelenskiy vowed that his forces would win back all the cities that had been lost to Russia, including Syevyerodonetsk.
The capture of Syevyerodonetsk, although now mainly a city of rubble, represents the biggest gain by Russian troops since they took the southern port of Mariupol, a city also left mainly in ruins after a bloody, protracted battle.
Syevyerodonetsk’s mayor said earlier in the day that the city was under full Russian control and that all exit routes to Ukrainian-held territory were blocked, leaving escape possible only through Russian-occupied areas.
"The city is now under the full occupation of Russia," Mayor Oleksandr Stryuk said on national television.
"They are trying to establish their own order. As far as I know, they have appointed some kind of commandant," he said, adding that it was "impossible" to leave the city to Ukrainian-held territory, stranding some 10,000 civilians.
Russia's Defense Ministry said that "as a result of successful offensive operations," Russian forces had established full control over Syevyerodonetsk and nearby towns and villages.
Russia continued to target areas across Ukraine with artillery and missile strikes, hitting military facilities in the west and north and continuing to pound key battleground cities in the eastern Donbas region.
Russia's revised military focus on Ukraine's east has brought Moscow closer to reaching its objective of capturing the Donbas, which is composed of the Luhansk and Donetsk regions.
Parts of both the Luhansk and Donetsk regions have been under the control of pro-Russia separatists since 2014, when Russia also invaded and annexed Ukraine's Crimean Peninsula.
The focus there was prompted by Moscow's failure to take the capital, Kyiv, in the first phase of the war following its February invasion.
The continued bombing of targets far from the front lines have led to accusations that Russia is trying to sow fear among civilians and draw neighboring Belarus into the conflict.
"48 cruise missiles. At night. Throughout whole Ukraine," Ukrainian presidential adviser Mykhaylo Podolyak wrote on Twitter on June 25. "Russia is still trying to intimidate Ukraine, cause panic and make people afraid."
In the west, officials in Lviv said Russian forces launched six missiles from the Black Sea and that four struck a military facility close to the Polish border.
Ukraine's northern military command wrote on Facebook on June 25 that 20 rockets that struck the Chernihiv town of Desna were fired from the air and from Belarusian territory, prompting Ukraine's intelligence service to accuse Russia of trying to drag Minsk into the war.
"Today's strike is directly linked to Kremlin efforts to pull Belarus as a co-belligerent into the war in Ukraine," the intelligence service said on Telegram.
Belarus has provided support for Russia's war effort in Ukraine but officially remains a nonbelligerent.
WATCH: Despite there being an exclusion zone around the Chernobyl nuclear plant since the catastrophic 1986 disaster, people live in the area. On the first day of the war they found themselves facing a new danger, as Russian tanks rolled through their villages -- and opened fire.
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Near Chernobyl, Residents Recall Brutality Of Russian Invasion
In a meeting in St. Petersburg with Belarusian leader Alyaksandr Lukashenka, Russian President Vladimir Putin said the Kremlin would supply ally Belarus with Iskander-M missile systems, a mobile guided-missile system with a range of up to 500 kilometers.
Putin also said Moscow would help Minsk upgrade its air forces in view of what Lukashenka called the "aggressive," "confrontational," and "repulsive" policies of neighboring Lithuania and Poland.
Hirske, a key district about 35 kilometers south of Lysychansk, was "fully occupied" by Russian forces on June 24, while officials reported the same day that Russian troops had taken control of Mykolaivka, situated near a highway to Lysychansk.
The General Staff of the Ukrainian Armed Forces said on June 25 that Ukrainian troops had repelled attacks near Bakhmut, which lies in the Donetsk region along an important supply route to Lysychansk.
Kyiv has received billions of dollars in aid from its Western partners since Moscow’s unprovoked invasion. On June 23, the United States announced $450 million in additional military aid for Kyiv, including four more HIMARS long-range multiple-rocket launchers, tens of thousands of rounds of artillery ammunition, and patrol boats.
Ukraine's leadership has expressed gratitude for the contributions -- and on June 25 the military said U.S. HIMARS were already being used effectively -- but they say much more is needed.
Fierce fighting has stretched both sides' personnel and equipment resources to the limit, with Kyiv repeatedly pleading with the West for more heavy weapons and Russia facing increasing difficulties in bringing qualified personnel to the front line.
Ukraine spy chief Budanov said the country’s “strategy is very simple. Stabilize the situation. Receive the required amount of equipment and prepare the required amount of forces and means to start the counteroffensive to return all our territory."
Budanov said Russia had committed 330,000 soldiers and noncombat personnel to its operations in Ukraine -- one-third of its total armed forces..
British Prime Minister Boris Johnson, an outspoken backer of Ukraine in its war with Russia, said he feared Kyiv could face pressure to agree to a “bad peace” deal with Moscow, a move that would lead to a long-term global “disaster.”
“Too many countries are saying this is a European war that is unnecessary ... and so the pressure will grow to encourage -- coerce, maybe -- the Ukrainians to a bad peace," Johnson told reporters during a visit to the Rwandan capital, Kigali, to attend a Commonwealth summit.
That would “be a disaster” and would “be a trigger for further escalation by Putin whenever he wanted,” Johnson said.