Up to 13,000 Ukrainian soldiers have been killed since the start of the Russian invasion in February, an official said on Thursday.
Mykhailo Podolyak, an adviser to the Ukrainian president, estimated that 10,000 to 13,000 servicemen and women have been lost to fighting.
Russia has remained tight-lipped about the number of its military personnel that have been killed. Moscow has put the figure in the thousands, though the UK Ministry of Defence estimates some 25,000 Russians have died.
Both sides are suspected of minimising their losses to not dent the morale of troops and the wider public.
“We are open in talking about the number of dead,” said Podolyak, adding that President Volodymyr Zelenskyy would make the official data public “when the time was right”.
Thursday’s figure is the first official estimate of the dead since August when the head of Ukraine’s armed forces said nearly 9,000 military personnel had been killed.
Two months before Zelenskyy said that “60 to 100 [Ukrainian] soldiers a day were killed in action”, with around 500 injured, when Russian forces were pushing to take over the Lugansk region in the east.
Ukraine’s position on the battlefield has now improved, with its armed forces making sweeping gains and recapturing the strategic southern city of Kherson in November.
In September, Russian Defense Minister Sergei Shoigu said 5,937 Russian soldiers had been killed since the start of the conflict on February 24.
This is far lower than estimates that have been put forward by western analysts.
US Chief of Staff, General Mark Milley, said in November that Russian losses were probably equivalent to those on the Ukrainian side, putting the figure at 10,000 Russian soldiers killed or injured.
Euronews cannot independently verify these figures.
The Ukraine war is the most deadly armed conflict to break out in Europe in decades, with several thousand civilians also dying as a result of the violence.