Serbian President Aleksandar Vučić said Russia should end efforts to recruit Serbs to fight alongside its Wagner paramilitary group in Ukraine.
He criticized Russian websites and social media groups for publishing Serbian-language ads in which the Wagner group calls for volunteers to join its ranks.
"It is not right that someone calls people in Serbia, which is the only one that has not imposed sanctions on Russia, to fight in Ukraine, when it is known that this is against our rules," the Serbian president said late Monday in an interview in Happy television.
Serbian volunteers fought alongside pro-Russian forces in Ukraine in 2014 and 2015. No one knows the exact numbers, but dozens of Serbs have signed up to fight in Ukraine since 2014, observers say.
On Tuesday, Russia's RIA news agency broadcast televised footage of two masked men identifying themselves as Serbian volunteers at a weapons training course in Russian-controlled parts of Ukraine's Zaporizhzhia region.
Serbian laws prohibit the participation of citizens of Serbia in conflicts abroad, and some people have been punished for this.
Last week, the high US envoy Derek Chollet expressed strong concern about the activities of the group of private military contractors "Wagner Group" and about the efforts that this group is making to recruit soldiers in Serbia and other countries.
The US State Department adviser said he expressed these concerns during talks in Belgrade with Serbian President Aleksandar Vucic.
"We have seen that the Wagner Group is trying to recruit soldiers from Serbia and elsewhere, and this is something that we think cannot continue," he told reporters on Thursday after meeting with the Serbian president.
The Serbian president denied allegations that the Wagner group, led by Evgeny Prigozhin, a close ally of Russian President Vladimir Putin, is present in Serbia, where pro-Kremlin and ultranationalist organizations have supported aggression against Ukraine.
He described as "lies" the assertions that members of the Wagner group were also seen in the north of Kosovo during the tensions of December, when groups of Serbs blocked the main roads in the area for about three weeks.
Serbia is a candidate to join the European Union, which is its main trading partner and investor, but also maintains commercial and military cooperation with Russia, a traditional ally, which supports Serbian claims to Kosovo.
Although it has repeatedly condemned Russia's aggression in Ukraine at the United Nations and several other international forums, Belgrade has refused to impose sanctions on Moscow.
Serbia is entirely dependent on gas imports from Russia and its oil supplier NIS is owned by Russia's Gazprom.