Russian forces are pounding Ukrainian positions with artillery fire and in the eastern region alone launched almost 400 strikes on Sunday, President Volodymyr Zelenskiy said in a video address.
Russia withdrew its forces from the southern city of Kherson this month and moved some of them to reinforce positions in the eastern Donetsk and Luhansk regions, an industrial area known as the Donbas.
"The fiercest battles, as before, are in the Donetsk region. Although there were fewer attacks today due to worsening weather, the amount of Russian shelling unfortunately remains extremely high," Zelenskiy said.
"In the Luhansk region, we are slowly moving forward while fighting. As of now, there have been almost 400 artillery attacks in the east since the start of the day," he continued.
Zelenskiy also said troops in the south were "consistently and very calculatedly destroying the potential of the occupiers" but gave no details.
Kyiv said on Saturday that about 60 Russian soldiers had been killed in a long-range artillery attack in the south, the second time in four days that Ukraine has claimed to have inflicted major casualties in a single incident.
Oleh Zhdanov, a military analyst based in Kyiv, said that according to his information, offensive battles were taking place on the Bakhmut and Avdiivka frontline in the Donetsk region, among others.
"The enemy is trying to break through our defenses, to no avail," Zhdanov said in a social media video. "We fight back - they suffer huge losses."
He said both armies are focused on regrouping. Ukrainian forces are being moved towards Zaporizhzhia, Dnipropetrovsk and Kharkiv, among others. Russian troops are regrouping to the Zaporizhzhia direction and Donetsk and Luhansk.
Russia's defense ministry said on Sunday that up to 50 Ukrainian servicemen were killed in the previous day along the southern Donetsk frontline and 50 others elsewhere. In total, 153 areas of concentration of Ukraine's manpower and military equipment were destroyed on Saturday.
Reuters was not able to immediately verify neither the Ukrainian nor the Russian reports.
Russia calls its invasion in Ukraine a "special operation" to demilitarize and "denazify" its neighbor, not a war. Kyiv and its allies say the invasion is an unprovoked war of aggression.
Kyrylo Tymoshenko, deputy head of Zelenskiy's administration, said late on Sunday that Russian forces fired at a residential building in the Kherson region, in southern Ukraine.
"There may be people under the rubble. Emergency services are working at the scene," Tymoshenko said on the Telegram messaging app.
The region's recently retaken capital, the Kherson city, remains without electricity, running water or heating.
(Additional reporting in Ottawa by David Ljunggren and in Melbourne by Lidia Kelly; Writing by David Ljunggren and Lidia Kelly;Editing by Sandra Maler and Diane Craft)
