Greece is allegedly about to expel two Russian diplomats for interfering in its internal affairs.
Athens is accusing Moscow of trying to undermine a recent deal over the name of its neighbour, the soon-to-be North Macedonia.
Russian news agencies have said Russia will respond in kind.
The incident - on the first day of the NATO summit - has raised some diplomatic eyebrows, since Greece is traditionally the alliance’s most pro-Russian member.
Al Jazeera's John Psaropoulos reports from Athens.
