A Europa League game had to be suspended for more than 20 minutes after a drone flew over the pitch.
The game between Luxembourg's F91 Dudelange and Azerbaijan's FK Qarabag was halted in the first half by the drone, which appeared to be carrying the flag of the disrupted region of Nagorno-Karabakh.
Azerbaijan and Armenia are in a territorial dispute over the region. The drone incensed Qarabag players, who tried to hit it with the ball as it flew over the centre circle.
Play later resumed, with Qarabag winning 4-1. UEFA said the decision to halt the match was taken "for safety and security reasons".
The European governing body said in a statement: "UEFA will await the official reports from the match officials to determine whether disciplinary proceedings will be opened".