Alexander Vershbow, Nato's deputy secretary general, told at the conference in Latvia: "Russian leaders are less and less able to conceal the fact that Russian soldiers are fighting - and dying - in large numbers in eastern Ukraine," Censor.NET reports citing BBC.
Alexander Vershbow said Russia's involvement was becoming more unpopular with the Russian public as a result.
There was "mounting evidence that the Russian incursion into Ukraine is becoming much less popular among the Russian public", he stated.
Russian officials dismissed on Thursday a US claim that Moscow had sent "thousands and thousands" of troops to fight alongside separatists.
A foreign ministry spokesman said the figures were "plucked out of the air".
Ukraine and the West have long accused Russia of helping the rebels with weapons and soldiers.
Independent experts echo that accusation but Moscow denies it, insisting that any Russians serving with the rebels are "volunteers".