Ukrainian and Russian troops battled in eastern regions on Thursday as Kyiv tried to push back occupying forces.
The Ukrainian military said the Russians were focused on an offensive in the Bakhmut sector of the Donetsk region, but their attacks in the Avdiivka and Kupiansk sectors were unsuccessful.
The governor of the neighbouring Luhansk region said Kyiv troops were recapturing areas there "step-by-step" but cautioned it was "not happening fast".
Luhansk and Donetsk make up the Donbas region, Ukraine's industrial heartland, parts of which were seized by Russian-backed proxies in 2014.
Russia unilaterally declared Donetsk, Luhansk, Kherson and Zaporizhzhia regions as part of its territory in September after sham referendums condemned by Ukraine and the West. Moscow does not fully control any of the four regions.
Bakhmut, which is now largely in ruins after months of battering by Russian artillery, is important because the Kremlin wants a relatively quick win to show to its public after a series of setbacks in the war.
It is located on a supply line between Donetsk and Luhansk. Gaining control of Bakhmut, with a pre-war population of 70,000-80,000 that has shrunk to close to 10,000, could give Russia a stepping stone to advance on two bigger cities -- Kramatorsk and Sloviansk.
Fighting has been particularly tough there, with commanders on both sides describing it as a "meat grinder".
Ukraine's military said it estimated 800 Russian soldiers were killed in the past day, mostly in fighting in Donetsk. The figure -- which would signify a huge loss of life for a single day -- could not be independently confirmed.
The Luhansk governor, Serhiy Haidai, said he expected fighting to intensify across the eastern front as temperatures drop
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