“We have official estimates from the General Staff (..) and they include between 10,000 and 13,000 dead,” Podolyak told Ukrainian TV channel 24.
Zelenskyi will release the official data “when the time is right,” Podolyak added.
In June, as Russian forces sought to completely capture the Luhansk region, Zelensky said Ukraine was losing “60 to 100 soldiers a day who are killed in action and about 500 people are wounded in action.”
Russian Defense Minister Sergei Shoigu announced in September that 5,937 Russian soldiers had been killed in the first seven months of hostilities.
Both sides are suspected of minimizing their losses to avoid demoralizing their soldiers.
US General Mark Milley said in November that more than 100,000 Russian servicemen have been killed or wounded in Ukraine and that Ukrainian forces may have suffered similar losses.
Thousands of Ukrainian civilians were also killed in the fighting, the worst in Europe for decades.