Kurt Volker, the U.S. special envoy for efforts to end the conflict in eastern Ukraine, met with Kremlin aide Vladislav Surkov on October 7 in Serbia's capital, Belgrade.
Serbian Foreign Minister Ivica Dacic said the two men held talks "dedicated to the Ukraine crisis" in remarks carried by state broadcaster Radio Television Serbia.
Dacic did not provide any other details and no specifics have emerged from the closed-door meeting.
The venue of the closed-door talks was not disclosed, although Serbian media said the two men met at a Belgrade hotel.
Volker tweeted on October 5 that he would meet Surkov on October 7 to discuss "how to catalyze Minsk implementation and restore Ukraine's territorial integrity."
"Minsk" refers to a February 2015 agreement, signed in the Belarusian capital, that called for a cease-fire and set out steps to end the conflict that have gone largely unimplemented.
The October 7 meeting was the second between Volker and Surkov, who is Russian President Vladimir Putin's point man for the conflict in eastern Ukraine.
Belgrade is a neutral venue. Volker had refused to meet with Surkov in Russia, and Surkov is barred from the European Union under sanctions imposed in response to Russia's aggression against Ukraine.
The two held talks for the first time on August 21 in Minsk.
U.S. Secretary of State Rex Tillerson appointed Volker, a former U.S. ambassador to NATO, as the U.S. special representative for Ukraine negotiations in July.
After the August meeting, Surkov said his discussion with Volker was "useful and constructive."
The war between Russia-backed separatists and government forces in eastern Ukraine has killed more than 10,000 people since April 2014.
Russia-backed separatists seized parts of the Donetsk and Luhansk provinces, which border Russia, and the war has persisted despite the February 2015 agreement and a September 2014 deal that was also signed in Minsk.