Since the beginning of the war between Russia and Ukraine in February nearly 13,000 Ukrainian troops have been killed, according to a senior adviser to President Volodymyr Zelenskyy.
"We have official estimates from the General Staff... and they range from 10,000 ... to 13,000 dead," Mykhailo Podolyak told Ukraine's Channel 24.
He added that the government will make the official data public "when the right moment comes".
In June, Zelenskyy said Ukraine was losing 60 to100 soldiers per day, and around 500 people wounded in action, as Russian forces battled to take full control of the easternmost Lugansk region.
In September, Russian Defence Minister Sergei Shoigu said 5,937 Russian troops had been killed in the nearly seven months of fighting to that point.
Both sides are suspected of minimising their losses to avoid damaging the morale of their troops.
Last month, US's top general Mark Milley estimated that more than 100,000 Russian military personnel have been killed or wounded in Ukraine, with Kiev's forces likely suffering similar casualties.
Those figures - which could not be independently confirmed -are the most precise to date from the US government.
Thousands of Ukrainian civilians have been killed in the worst fighting in Europe in decades.
According to the Office of the High Commissioner for Human Rights in October, there have been 15,246 civilian casualties in Ukraine from February to October. That includes 6,114 killed and 9,132 injured.