A top European Union official blasted Russia for reviving threats of retaliation against Ukraine over a trade deal with the bloc, stoking political tensions even as signs mount of a military de-escalation in the conflict zone.

The North Atlantic Treaty Organization said that Russia had withdrawn a sizable number of troops from eastern Ukraine—though some remained. Meanwhile Russia-backed rebels in the region said they had begun pulling back their heavy artillery, after Ukrainian troops did the same.

Scattered fighting continued, however, amid mutual accusations of violations of a cease-fire that took effect 2½ weeks ago—indicating any peace taking hold remains fragile as Kiev continues to challenge Moscow by seeking closer ties to Europe.

In a three-way deal reached Sept. 12, the EU and Ukraine agreed to postpone part of the trade pact, including dropping tariffs on EU goods, until 2016.

Russia argued the pact would lead to a flood of EU imports entering Russia and undercut Russian exports to Ukraine. It had threatened to retaliate by ending its preferential trade relationship with Kiev, a move that could deliver a fresh blow to Ukraine's tottering economy.