Nato's deputy chief says Russian leaders are less and less able to conceal the deaths of "large numbers" of Russian soldiers in eastern Ukraine.
Alexander Vershbow said Russia's involvement was becoming more unpopular with the Russian public as a result.
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".
Alexander Vershbow, Nato's deputy secretary general, told a 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."
There was "mounting evidence that the Russian incursion into Ukraine is becoming much less popular among the Russian public", he added.
Earlier this week, Russian investigative newspaper Novaya Gazeta published an interview with a wounded tank gunner, who said that he was a regular Russian army soldier and had been fighting alongside rebels.
He said he had received thorough training in Russia before being sent to Ukraine.
Earlier, Ukraine's parliament passed a new law to increase the size of the country's armed forces to counter the threat from pro-Russian separatists in the east.
The bill calls for a force of 250,000 personnel, including 204,000 active troops - up from 184,000. It will now go to President Petro Poroshenko, who proposed the measure, to be signed into law.
Lull in fighting
Meanwhile the government said its army had begun the second stage of pulling back heavy weapons on Thursday in line with the ceasefire agreed in Minsk last month.
A lull in fighting has led to hopes that the truce may broadly be holding.
Ukrainian military spokesman Vladislav Seleznyov said on Thursday one soldier had been killed on the frontline in the past day.
Rebels battled government troops in the village of Shyrokyne, east of Mariupol, in the early hours, Ukrainian interior ministry adviser Zoryan Shkiryak said. But he added that there had been no other serious fighting in the area.
Both Ukraine and the rebels say they are withdrawing heavy weapons from the front line, one of the requirements of the ceasefire deal made in the Belarusian capital.
Monitors from the OSCE security group have reported weapons movements on both sides but say it is too early to confirm a full withdrawal.
In other developments:
- A lawyer for the Ukrainian pilot Nadiya Savchenko - awaiting trial in Moscow - has confirmed she has decided to drink broth, so breaking her 83-day hunger strike
- Reports say a brawl broke out in a casino in the southern Ukrainian city of Odessa after pro-Kiev activists belonging to the right-wing group Pravy Sektor (Right Sector) attempted to close it down
- Italian Prime Minister Matteo Renzi discussed the crisis in Ukraine with President Vladimir Putin during a visit to Moscow
- UK Foreign Secretary Philip Hammond met his counterpart Pavlo Klimkin in Kiev, and reaffirmed a commitment to provide British military trainers to Ukraine
The UN says the conflict has claimed at least 6,000 lives since it started in April, although it says that the real number of fatalities could be considerably higher.
On Tuesday, the US army's Europe commander, Ben Hodges, said the US believed around 12,000 Russian soldiers were supporting the separatists.
But US Assistant Secretary of State Victoria Nuland on Wednesday told a congressional foreign affairs committee that Russia had deployed "thousands and thousands" of troops, without giving a precise number.
Russian foreign ministry spokesman Alexander Lukashevich rejected the figures on Thursday, saying they were "plucked out of the air".
Meanwhile, Russia's defence ministry said it had started military exercises involving some 2,000 troops in southern Russia, the breakaway Georgian territories of Abkhazia and South Ossetia and in Crimea, which was annexed by Russia from Ukraine last year.