Serbian President Aleksandar Vucic has praised Russia for sending sophisticated antiaircraft missiles to joint air defense drills.
Vucic, visiting a military airport near Belgrade, said he was impressed by what Russia's surface-to-air systems could do, adding that he was "thankful to our Russian friends for drastically boosting our defense capabilities."
The Russian Defense Ministry said that the long-range S-400 and the short-range Pantsir-S systems were taking part in the Slavic Shield-2019 exercise to test joint action by the Russian and Serbian militaries to fend off air attacks.
The S-400 is to be returned to Russia after the six-day drills, while Vucic said the Pantsir system was purchased by his government.
Serbia maintains strong political and economic relations with Russia despite a proclaimed goal of joining the EU. Belgrade has pledged to stay out of NATO and refused to join Western sanctions against Russia over Ukraine.
Serbia was also due on October 25 to sign a free trade agreement with a Russian-led economic bloc despite warnings from the EU that it could jeopardize its membership bid.
"We are talking about fantastic systems," Vucic said after attending a computer-simulated antiaircraft drill. "If we had S-400, no one would dare overfly Serbia."
NATO bombed Serbia in 1999 to curb a bloody crackdown against Kosovo Albanians.
Kosovo declared independence from Serbia in 2008, a move Belgrade and Moscow don't recognize.
Russia has been helping to beef up Serbia's military with fighter planes, attack helicopters, and battle tanks.