Serbia has been using military equipment and weapons supplied by Russia while taking part in one of the largest NATO exercises in more than a decade. Adding to its large cache of Russian weapons already used by the country’s military, Serbian Prime Minister Aleksander Vucic signed a deal to buy more arms from Moscow after meeting last week with his Russian counterpart, Prime Minister Dmitry Medvedev, according to a report Monday by Russian news site Sputnik. The two countries share close political and cultural ties.

Incidences of alliance training partners using Moscow-supplied weaponry are relatively rare after large parts of Eastern Europe were freed from the grip of the Soviet Union at the end of the Cold War in 1991. But Serbia's insistence on continuing to buy Russian wares sheds light on its unique foreign policy that allows it to pursue long-term European ambitions and keep its close relationship with Russia.

Experts say Serbia’s military relationship with Russia will not hurt its plans to be part of the European Union, but warn that attempting to remain political and military allies with Moscow and the West cannot be a long-term policy. Serbia’s decision to buy Russian arms will not be a huge concern for the EU, as Serbian officials have repeatedly said that European membership remains the strategic goal, Zoran Dragisic, a security studies professor in Belgrade, told Balkan Insight, an independent news website covering the Balkans.

"One should not forget that even some NATO country members, which were members of the Warsaw pact, still have Russian weapons as well. Replacing the weapons is an expensive and timely process. Bulgaria, Hungary and the Czech Republic have yet to do so, therefore no one can expect Serbia to do it," Dragisic said. "Serbia is not a member of the EU at the moment and is trying to sit in two chairs. I do not think it is possible in the long run.”

The Warsaw Pact was a Soviet equivalent to NATO before the Soviet Union collapsed in 1991.

The weapons deal was signed just as Serbia troops prepared to conduct the huge U.S.-led NATO military drill in Germany that began last week and will run until Nov. 6.