Russia's Defense Ministry says its troops massed along the Ukrainian border are packing up and moving to train stations and airfields as they return to permananent bases.
President Vladimir Putin said on May 21 he ordered the troop withdrawal to help create "favorable conditions" for the May 25 Ukrainian presidential election.
Ukrainian officials say Russian troops have moved at least 10 kilometers away from Ukraine’s border. But Kyiv could not confirm Moscow's claims of a full withdrawal.
NATO Secretary-General Anders Fogh Rasmussen said on a visit to Sarajevo on May 21 that there was no "firm evidence" of a pullback.
He added that this was the third time Putin has announced a troop withdrawal from the Ukrainian border.
Rasmussen said a pullout would be an "extremely important step to de-escalation" of the crisis in Ukraine.
White House spokesman Jay Carney said the United States had seen "some indications of activity on the border" but added it was "too early" to conclude that a withdrawal was under way.
Putin, speaking in Shanghai at a security summit, said those who hadn't seen signs of a pullout "should look better."
He added that if the weather was clear, Western countries should be able to see the withdrawal via satellite photos.
NATO estimates Russia has some 40,000 troops near Ukraine.
Lieutenant General Mark Schissler, the deputy chairman of NATO's Military Committee, said on May 21 that even if there was a pullback of Russian forces, the Atlantic alliance "will have to reset to a new reality."
"You can't set the clock back and pretend nothing happened in the last two months...the world is now a different place because of what's gone on." Schissler added.
'National Dialogue' Continues
Meanwhile, top Ukrainian officials on May 21 held "national dialogue" talks in the southern city of Mykolayiv aimed at ending a pro-Russian separatist insurgency in eastern Ukraine.
The first two rounds of talks made no tangible progress and did not include any representatives from the two self-declared "people's republics" that were proclaimed in the Donetsk and Luhansk regions by pro-Russian separatists.
Skirmishes between Ukrainian forces and separatist gunmen have occurred in recent days, particularly around the eastern city of Slovyansk, a rebel stronghold.
The outskirts of the city -- which is encircled by Ukrainian troops -- was visited on May 21 by acting Ukrainian President Oleksandr Turchynov.
He said people in eastern Ukraine were beginning to "understand that the separatist terrorists are leading the region to the abyss."
With reporting by Reuters, AP, and ITAR-TASS