Limited Russian troop movements near the border with Ukraine "may suggest" preparations for a withdrawal, NATO chief Anders Fogh Rasmussen said on Thursday.

"Late yesterday (Wednesday), we have seen limited Russian troop activity in the vicinity of the border with Ukraine that may suggest that some of these forces are preparing to withdraw," Rasmussen said in Montenegro.

"It is too early to say what this means, but I hope this is the start of a full and genuine withdrawal," he said.

President Vladimir Putin on Monday announced that Russian troops near the border, estimated by NATO to number 40,000, were to return to bases after the end of spring exercises.

Their presence had raised deep concerns after Moscow's annexation of Crimea in March and an uprising by pro-Moscow rebels in eastern Ukraine.

Kiev said on Thursday at least 14 Ukrainian soldiers had been killed by separatist rebels, dealing a heavy blow to the beleaguered government just three days before a crunch presidential poll.

The Pentagon also said there were signs of a possible withdrawal.

"We've seen some Russian forces move off the border," Colonel Steven Warren told reporters.

But he added: "What's important is that the Russians still maintain a very capable and very threatening force along the Ukrainian border."

Russia's defence ministry said Thursday that four trains and more than a dozen planes had taken equipment and troops away from the area.

"At present, most of the previously deployed Russian force remains near the Ukrainian border and we see continued Russian exercises in the same area," Rasmussen said.

"If we see any meaningful, comprehensive and verifiable withdrawal, I would be the first to welcome it.

"This would be a first step from Russia into the right direction of living up to its international commitments, especially as Ukraine is preparing to hold important presidential elections on Sunday," he added.

In Brussels, NATO's top military commander, US General Philip Breedlove, said it was too early to characterise "some movement" of troops, which was taking place in just one area along the border.

"The force that remains is very large and remains in a very coercive posture," Breedlove said, stressing that there needed to be "100 percent pullback".

"The scope of the movement seen so far is not going to affect the capability of the force there," he said, adding that Russia's actions in Ukraine had completely changed the European security situation.

"We are at a crossroads on how we will adapt ... in the long-term to address an aggressive neighbour."

Both NATO and Washington had earlier said they saw no evidence of any withdrawal.