Belgrade has launched a probe after a video emerged purporting to show a Russian diplomat or intelligence officer handover a wad of cash to a Serbian man.
Serbia's president, Aleksandar Vucic, ordered the investigation after the footage was published to YouTube on November 17.
Serbia's intelligence agency believes the video is authentic and plan to release further information following a national security council meeting on Thursday (November 21), according to Tanjug, the state news agency.
News of the probe prompted Moscow to play down concerns of weakening diplomatic relations with Belgrade.
The footage, which is the only upload to an account named Kdjuey Lskduf, purports to show two men meeting before having a drink at a Black Sheep Brewpub (Crna Ovca) in Serbia.
One of the men, who is said to be a Russian diplomat or intelligence officer, is seen handing over a carrier bag to the second man, whose face is blurred in the video but is said to be Serbian, according to the uploader of the footage.
The second man later gets into a vehicle, where he is seen rummaging through the bag and pulls out a wad of cash.
Online critics claim the video is fake.
Euronews is working to verify the authenticity of the video. Frames from it have not appeared elsewhere online.
Serbian Prime Minister Ana Brnabic told Blic that such an incident would be "serious matter and problem" if it were proven to be true.
Serbian relations with Moscow are known to be generally quite close, with the former relying on the latter for oil and gas, and for sharing technology for its armed forces.
Last month Serbia signed a trade deal with the Russian-led Eurasian Economic Union despite being a candidate for EU membership.
The recent blocking of EU accession talks opening with Albania and North Macedonia could turn Balkan heads towards Moscow.