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.
The European Union and the United States later imposed sanctions against Russia over its role in the Ukraine conflict.
"There has been a significant pullback of Russian conventional forces from inside Ukraine, but many thousands are still deployed in the vicinity of the border," Canadian Lt Col Janzen said on Wednesday.
"Some Russian troops remain inside Ukraine. It is difficult to determine the number, as pro-Russian separatists control several border crossings and troops are routinely moving back and forth across the border.
"Further, Russian special forces are operating in Ukraine, and they are difficult to detect," Lt Col Janzen added.
Ukrainian officials have repeatedly stated that thousands of heavy-armed Russian troops remain positioned on Ukraine's eastern border.
On Wednesday, Ukrainian military journalist Dmytro Tymchuk said a Russian troops build-up also continued in northern Crimea, where "several air defence systems were moved".
Meanwhile US President Barack Obama said that Russia's "actions in Ukraine challenge this post-war order".
Speaking at the UN General Assembly, he said "America and our allies will support the people of Ukraine as they develop their democracy and economy", urging Russia to change course.
And he stressed that if Russia took "the path of peace and diplomacy", then the US would lift its sanctions.
On Tuesday, rebel leaders said they would hold elections on 2 November, in a show of defiance towards the central government in Kiev.
Ukraine's parliament passed a bill last week granting three-year "self-rule" to parts of the Donetsk and Luhansk regions, with elections on 7 December - a decision condemned by some MPs as "capitulation".
But the rebels said they would set a preliminary date five weeks earlier. They both aim to elect a Supreme Soviet (parliament) and their own leaders.
Ukraine's parliamentary elections are set for 26 October.
Under the terms of a nine-point deal agreed at the weekend in Minsk, Belarus, Ukrainian government and rebel forces have agreed to pull their big guns 15km back from the line of engagement, creating a 30km buffer zone.
Rebels in Donetsk began moving their heavy artillery in some areas on Tuesday, according to Donetsk separatist leader Alexander Zakharchenko, "but only in those areas where the Ukrainian regular units have done the same".
Ukraine announced on Monday that it was preparing to move its heavy weaponry, although all other units and their weapons would remain in place.
Minsk memorandum: Key points
- To pull heavy weaponry 15km by each side from the line of contact, creating a 30km security zone
- To ban offensive operations
- To ban flights by combat aircraft over the security zone
- To set up an OSCE monitoring mission
- To withdraw all foreign mercenaries from the conflict zone