Russian paratroopers have launched joint exercises with Serbian military units at the same time as NATO is holding exercises in neighboring Montenegro.
The competing training sessions come amid growing suspicions about Russia's intentions in the Balkans, where Moscow opposes Montenegrin membership in NATO and Serbia remains one of the Kremlin's staunchest allies.
Serbian Defense Ministry spokesman Jovan Krivokapic said the drills with Russia, which started on November 2 and are called Slavic Brotherhood, were aimed at improving joint responses for counterterrorism operations.
Several dozen Belarusian troops are also involved in the maneuvers.
A day earlier, teams from 32 NATO members and partner countries began drilling responses to natural disasters and a simulated chemical weapons attack in neighboring Montenegro.
"These exercises...show how strong is the Euro-Atlantic relationship, how it exists as something that connects people, strengthens their solidarity and commitment, and this relationship is based on these values," said John Manza, NATO deputy assistant secretary-general for operations.
Earlier this year, NATO formally invited Montenegro to join the alliance, prompting an angry response from Russia.
Last week, Montenegrin Prime Minister Milo Djukanovic abruptly resigned after suggesting that Russia was involved in an alleged coup attempt and accusing the opposition of collaborating with the Kremlin.