MiG-29 jet fighters of Russian aerobatic team Strizhi (Swifts) perform during ceremony in Batajnica, military airport near Belgrade, Friday, Oct. 20, 2017. 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. (AP Photo/Darko Vojinovic)

MiG-29 jet fighters of Russian aerobatic team Strizhi (Swifts) perform during ceremony in Batajnica, military airport near Belgrade, Friday, Oct. 20, 2017. 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. (AP Photo/Darko Vojinovic)

BATAJNICA, Serbia (AP) — Russia formally handed over six MiG-29 fighter jets to Serbia Friday, part of an arms delivery that has the potential to accentuate tensions in the war-weary Balkans.

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

“Thank you for these beautiful gifts,” Vucic told Shoigu at the ceremony, which was accompanied by an air show and attended by tens of thousands of visitors.

“These planes are not a threat to anyone, but show that no one can attack Serbia unpunished,” he said.

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

Shoigu said after arriving in Belgrade that the jets will provided a “safe-shield and the guarantee of Serbia’s security and independence.”

Russia has also promised the delivery of 30 battle tanks and 30 armored vehicles to Serbia, which was at war with neighbors Croatia, Bosnia and Kosovo in the 1990s during the bloody breakup of the former Yugoslavia.

Serbia has been on the path to join the European Union, but under 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.

Serbia, which claims military neutrality, is a member of NATO’s outreach Partnership for Peace program and has held military exercises with both the Russians and the Western military alliance.

The air show that marked the liberation of Belgrade in World War II by the Red Army and Yugoslav troops also featured the Russian aerobatic team Strizhi, an anti-terrorist drill and a parade of Serbia’s elite infantry units.