The latest figures follow a lightning counteroffensive in Ukraine’s northeast and south that won back large tracts of territory.
As many as 13,000 Ukrainian soldiers have been killed in battle since Russia invaded the country nine months ago, according to an official in Kyiv.
The comments from Mykhailo Podolyak, an adviser to Ukrainian President Volodymyr Zelenskyy, appeared to be the first update on the number of dead fighters since late August when the head of the armed forces said nearly 9,000 troops had been killed.
“We have official figures from the general staff, we have official figures from the top command, and they amount to (between) 10,000 and 12,500 to 13,000 killed,” Podolyak told Ukraine’s Channel 24 on Thursday.
“We are open in talking about the number of dead,” he added, saying more soldiers had been wounded than had died.
The number of military casualties has not been confirmed by Ukraine’s armed forces.
Ukraine’s military mounted a lightning counterattack in September that saw them win back swathes of territory in the northeast and south of the country, including the strategically-important city of Kherson which had been occupied by Russia shortly after its February 24 invasion.
With the weather turning colder, the most intense clashes are now taking place in the eastern region of Donetsk.
Earlier, United States President Joe Biden and his French counterpart Emmanuel Macron, who is in Washington, DC, promised to maintain support for Ukraine and condemned “Russia’s illegal war of aggression” against its neighbour.
Last month, the US’s top general estimated that Russia’s military had seen more than 100,000 of its soldiers killed or wounded in Ukraine, adding that Kyiv’s armed forces have “probably” suffered in a similar way.
Zelenskyy adviser Oleksiy Arestovych, speaking in a video interview on Wednesday, said the Russian death toll was about seven times that of Ukraine’s.