Russian ally Serbia took the delivery of a sophisticated Chinese anti-aircraft system in a veiled operation this weekend, amid Western concerns that an arms buildup in the Balkans at the time of the war in Ukraine could threaten the fragile peace in the region.

Media and military experts said Sunday that six Chinese Air Force Y-20 transport planes landed at Belgrade’s civilian airport early Saturday, reportedly carrying HQ-22 surface-to-air missile systems for the Serbian military.

The Chinese cargo planes with military markings were pictured at Belgrade’s Nikola Tesla airport. Serbia’s defense ministry did not immediately respond to AP’s request for comment.

The arms delivery over the territory of at least two NATO member states, Turkey and Bulgaria, was seen by experts as a demonstration of China’s growing global reach.

Serbian President Aleksandar Vucic all but confirmed the delivery of the medium-range system that was agreed in 2019, saying on Saturday that he will present “the newest pride” of the Serbian military on Tuesday or Wednesday.

He had earlier complained that NATO countries, which represent most of Serbia’s neighbors, are refusing to allow the system’s delivery flights over their territories amid tensions over Russia’s aggression on Ukraine.

The Chief of Staff of the Presidency of Kosovo, Blerim Vela, reacted after the publication of the Associated Press research.

“Second day of shipments of Chinese arms to Serbia: several flights of Chinese Air Force flew over Bosphorus en route to Batajnica Air Base to deliver Guizhou Aerospace Industry Shaanxi Yellow River Group manufactured HQ22 Red Banner SAM system’s export version FK3”, he added.

Although Serbia has voted in favor of U.N. resolutions that condemn the bloody Russian attacks in Ukraine, it has refused to join international sanctions against its allies in Moscow or outright criticize the apparent atrocities committed by the Russian troops there.