It is about weapons systems used in the joint Russian-Serbian military exercise, which in English is called the Slavic shield in Serbia over the past week. It is the first time, among other things, that the S-400 air defense robot system participates in exercises outside Russia, according to the Moscow Ministry of Defense.

It has been used in Syria, and Turkey has ordered the S-400 despite opposition from its NATO allies.

"Careful and cautious"

US Balkan broadcaster Matthew Palmer tells Northern Macedonian TV that the US is concerned about Russian weapons systems being placed on Serbian territory and the possibility that Serbia is considering buying them.

"We hope our Serbian partners are careful and cautious with such transactions," he says.

-We have a strong military partnership with Serbia. I think the United States is actually Serbia's best military partner, he adds.

Close ties with Russia

Aleksandar Vucic comments on the statement for Serbian media.

"We will, of course, be cautious, because I do not want Serbia to be subject to sanctions from the leading world power, regardless of the fact that they would be completely unjustified and unfair," he says.

Serbia has historically close ties with Russia, which is keen to maintain influence in the Balkans, where NATO is gaining ground. Slovenia, Croatia, Montenegro and Albania are part of the Western Defense Alliance and Northern Macedonia is expected to become a member next year.