Ukraine says about 10,000 of the country's soldiers have died in the war since the beginning of the Russian occupation in late February.
According to foreign media, the Telegraph reports, presidential adviser Oleksiy Arestovych and details in a social media interview, when asked how many troops Ukraine had lost in the first 100 days of the war - that is, until June 3.
Ukraine says it is losing about 100 soldiers every day in fighting, with hundreds more wounded.
Arestovych said Russia's losses were several times greater.
The Ukrainian government says more than 30,000 Russian troops have died.
However the intelligence rating in the UK is lower - more than 15 thousand.
Russian authorities have not provided any official figures for their war losses in Ukraine since March 25, when they said 1,351 soldiers and officers had died.
BBC Russian has confirmed the deaths of 3,502 Russian soldiers - but that only covers those who have been named and buried in cemeteries.
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