President Volodymyr Zelenskyy has urged Russian troops to flee from an offensive launched by Ukraine’s forces near the southern city of Kherson, saying they were taking back their territory.
Most of the Kherson region bordering the Black Sea, and its provincial capital of the same name, were seized by Moscow’s military at the start of the invasion six months ago.
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Fighting was raging across almost the entirety of the region on Tuesday, Ukraine’s presidency said.
As often the case throughout the conflict, Russia dismissed Kyiv’s claims, saying the assault had failed.
With the war in the eastern Donbas region largely stalled, analysts have said for weeks that combat is likely to shift south to break the stalemate before winter comes.
Ukraine’s assault comes after weeks of a deadlock in the conflict that has killed thousands, displaced millions, destroyed cities and caused a global energy and food crisis.
It has also fuelled worries of a radiation disaster being triggered by shelling near the Zaporizhzhia nuclear plant in southern Ukraine.
In his nightly address late on Monday, Zelenskyy vowed that Ukrainian troops would chase the Russian army “to the border”.
“If they want to survive – it’s time for the Russian military to run away. Go home,” he said. “Ukraine is taking back its own.”
Oleksiy Arestovych, a senior adviser to the Ukrainian president, said Russian defences had been “broken through in a few hours”.
Ukrainian forces were shelling ferries that Russia was using to supply a pocket of territory on the west bank of the Dnieper River in the Kherson region, he added.
While Russia’s defence ministry said Ukraine’s military had attempted an offensive in the Mykolaiv and Kherson regions, it alleged that Kyiv’s forces sustained significant casualties, according to the RIA news agency.
The “enemy’s offensive attempt failed miserably”, it said.
But a Ukrainian barrage of rockets left the Russian-occupied town of Nova Kakhovka, about 55km (34 miles) northeast of Kherson, without water or power, officials at the Russian-appointed authority told RIA.
Al Jazeera could not independently verify the battlefield reports.
Also on Tuesday, fresh Russian strikes in the northeast city of Kharkiv killed at least five people, prompting officials to urge people to stay indoors.
But much of the attention remained on the counter-offensive in the south.
“Powerful explosions continued throughout the day and throughout the night in Kherson region,” Zelenskyy’s office said in a morning update on Tuesday.
“The Armed Forces of Ukraine launched offensive actions in various directions,” it added.
Regional officials said the long-awaited offensive was “the beginning of the de-occupation of Kherson region”.
In an intelligence note, Britain’s defence ministry said although it was “not yet possible to confirm the extent of Ukrainian advances”, its army had increased “artillery fire in front line sectors across southern Ukraine”.
It added that it was using “long-range precision strikes to disrupt Russian resupply” lines.