A drone carrying a flag of a disputed territory hovered over the pitch during Dudelange's meeting with Azerbaijani side Qarabag.
Dudelange have apologised to Qarabag after a drone carrying the flag of a region disputed by Azerbaijan and Armenia caused Thursday's Europa League match to be temporarily suspended.
Play was halted in the first half of the Group A encounter in Luxembourg when a drone hovered above the pitch with the flag of the Nagorno-Karabakh region, which is derived from the Armenian flag.
Both sets of players attempted to knock the drone out of the air by kicking balls at it, with the match delayed by over 15 minutes.
When the match resumed, Qarabag took control, gaining a 3-0 lead by half-time with Dani Quintana's goal adding further gloss after the interval.
Qarabag originated in Agdam, a now ghost-town in south-west Azerbaijan that was destroyed by Armenian forces in the 1990s.
Nagorno-Karabakh and some surrounding territory has been under Armenian control after a 1994 ceasefire brought a six-year war to an end. Azerbaijan and Armenia have no diplomatic relations.
Dudelange swiftly issued an apology while vowing to investigate the incident.
"We apologise to [our] guests from Azerbaijan," the club tweeted. "We have nothing to do with this provocation. We'll figure [it] out!"
It is not the first time tensions between the neighbouring nations of Azerbaijan and Armenia have caused controversy in the Europa League.
Armenia star Henrikh Mkhitaryan elected not to travel to Baku for the 2018-19 final against Chelsea in May, having also sat out a 3-0 away win over Qarabag in last season's group stage.