The criticism marks a rare public rebuke from the Serbian leader toward Russia -- a steadfast ally of the Balkan country.

"Why do you do that to Serbia?" Vucic said during a televised interview late Monday, according to the Beta news agency.

"Why do you, from Wagner, call when it is against the regulations?"

The controversial advertisement appeared earlier this month in the Russian state media outlet RT's Serbian affiliate.

Small numbers of Serbians have fought alongside Russian-backed forces in Ukraine since fighting first broke out in the country in 2014.

The exact number of Serbs who fought in Ukraine has never been disclosed by officials.

Serbia has long been a reliable ally to Moscow, with shared Orthodox heritage, mutual hatred of NATO, and military alliances during several wars strengthening their relations.

Serbia remains the only European country -- apart from Belarus -- that did not join Western sanctions against Moscow.

On Tuesday, Russia's RIA news agency published footage apparently showed two Serbian citizens participating in a weapons training course in Ukraine.

The Wagner mercenary outfit founded in 2014 -- which has been involved in conflicts in Africa, Latin America and the Middle East -- shot to prominence after President Vladimir Putin sent troops into Ukraine on February 24, 2022.

Tens of thousands of Russians have relocated to Serbia since the outbreak of war, where most have been warmly welcomed.

Despite the arrival of dissident Russians fleeing the conflict, Serbians by and large remain ardent supporters of the invasion of Ukraine, with pro-Kremlin rallies held in the capital Belgrade.