Nato says it has observed a “significant” withdrawal of Russian troops from eastern Ukraine, but adds that some forces still remain there.
Nato spokesman Lt Col Jay Janzen says it is difficult to determine the number of soldiers in Ukraine, as pro-Russian rebels control some border crossings.
Moscow denies arming the rebels and sending Russian troops to the Donetsk and Luhansk regions.
UN officials say 3,245 people have been killed since fighting began in April.
A shaky ceasefire has held since 5 September, and the two sides have since agreed to set up a 30km (19 miles) buffer zone.
But there were reports of overnight shelling in Donetsk, Avdiivka and Debaltseve.
In March, Russia annexed Ukraine’s Crimea southern peninsula – a move condemned by Ukraine and the West.