There is "heavy fighting" in "almost the entire territory" of the southern Ukrainian region of Kherson occupied by Russian troops, Ukraine's presidency said Tuesday.
The clashes were raging after Ukrainian troops on Monday launched a counter-offensive to retake the southern region which was seized by Russia early in the war.
"Powerful explosions continued throughout the day and throughout the night in Kherson region," the president's office said in a morning update.
"Heavy fighting is taking place in almost the entire territory of the Kherson region. 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.