The Serbian president has said his country's security services have uncovered spying by Russian agents.
Aleksandar Vucic confirmed a Russian intelligence official had a meeting with a retired Serbian military officer during which money changed hands.
A YouTube video of the December meeting surfaced at the weekend.
Mr Vucic said his officials had also found evidence of 10 other contacts between Russian agents and Serbian officers.
But he insisted these incidents would not affect Serbia's close relations with Russia. He said he was sure President Vladimir Putin was not informed about the actions of the Russian agents.
What does the video show?
The video appears to show a Russian intelligence officer giving money to a Serbian man in Belgrade. The pair also visit a bar.
The Russian is identified as Lt-Col Georgy Kleban, a former assistant military attache at Russia's embassy in Belgrade, greeting a retired Serbian army officer identified only as ZK.
The video was not made by Serbian agents and the president did not disclose who filmed it.
What did the president say?
"On several occasions, Serbian security agencies gathered audio and video evidence of contacts between Lt-Col Kleban and members of the Serbian army," Mr Vucic told a news conference.
Efforts to carry out video surveillance of the pair by Serbian agents were hampered due to the Russian having his own protection detail, he added.
But Serbian agents had evidence concerning other Russian spies, the president said.
"There have been 10 contacts with three sources," he said.
He insisted the episode would not sour strong relations between the allies.
"We will not change our policy towards Russia, which we see as a brotherly and friendly country... but we will strengthen our own intelligence defences," Mr Vucic said.
"There was only thing I have asked the Russian ambassador today - I've only asked him - Why?"