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Russia has formally handed over six MiG-29 fighter jets to Serbia, part of an arms delivery that could worsen tensions in the war-weary Balkans.

The ceremony Friday at a military airport close to the Serbian capital, Belgrade, was attended by Russian Defense Minister Sergei Shoigu and Serbian President Aleksandar Vucic.

Moscow is handing over the MiGs for free, but it is estimated that the overhaul of the secondhand aircraft will cost Serbia about 200 million euros ($235 million.) The fighter jets are to enter service next year.

Serbia has been on the path to join the European Union, but under political and propaganda pressure from Moscow has slid toward the Kremlin and its goal of keeping the countries in the Balkan region out of NATO and other Western bodies.