Serbia's unique role as an ally to both NATO and Russia will face another test later this year when the Balkan nation holds joint air force exercises with Moscow. The combined military training called "Slav Brotherhood 2016" was announced Monday and will also include Russia's ally, Belarus, Reuters reported. 

"It represents the continuation of the cooperation between the two air forces that began in 2014 with a joint military exercise," the defense ministry said Monday. Serbia and Russia will later hold another military exercise without Belarus later this year. 

Serbia has juggled its commitments to the U.S.-led NATO and Russia for years, even as Moscow and Washington have increasingly exchanged heated rhetoric over missile defense, territory and other delicate topics. After Russia annexed Crimea from Ukraine in 2014, the European Union and United States levied sanctions against Moscow, which responded with its own sanctions.

Serbia has expressed interest in joining the European Union and has partnered with NATO in the past, but the military alliance remains unpopular at home after a 1999 bombing campaign forced the Serbian military out of Kosovo. While Serbia also relies on Russia for energy, it counts the European Union as its top trade partner and benefactor.

"Serbia has demonstrated great enthusiasm for the partnership with NATO, relations between NATO and Serbia will be developing, benefits for Serbia will increase," Gordon Duguid, deputy chief of the United States mission to Serbia, told the Tanjug news agency in June. 

Russia also maintains a strong military alliance with Serbia. Belgrade in 2012 allowed Moscow to open a base in Serbia's southern city of Nis, while Serbian military rely on Russian technology. After visits from Serbian President Tomislav Nikolic and Prime Minister Aleksandar Vucic to Moscow, large-scale weapons purchases were announced. 

Serbia has largely tried to appear independent from Russia in the past. The Serbia military has held 21 multinational training exercises, but only two were with Russia.

"Serbia's goal is to join the European Union and the process will also include the EU's Common Security and Defense Policy agenda. Most NATO policies fit in that agenda," a Serbian defense official recently told Reuters on condition of anonymity. "Russia remains a partner, we will not join NATO, but our road heads to the West." he said.