The Russian advance on Kharkiv came at a "high price" for its troops, a NATO official has told European Pravda.
The official, who spoke on condition of anonymity, said: "Russia likely suffered losses of almost 1,000 people a day in May, which is quite an astronomical figure."
They were referring specifically to fatalities.
Russian forces have advanced less than 10km into Ukrainian territory and have not taken control of the city of Vovchansk.
The US defence secretary earlier said Russia's advance was slowing and the frontline stabilising, partly thanks to countries lifting restrictions on Ukraine's use of donated weapons on Russian territory.
