Belgian authorities launched an investigation on Wednesday after drone sightings forced the suspension of flights overnight at Brussels and Liege airports, leaving hundreds of passengers stranded.

Both airports halted arrivals and departures late Tuesday following reports of drones near their airspace, prompting the cancellation of dozens of flights and forcing around 500 passengers to spend the night at Brussels Airport.

Belgium’s Interior Minister Bernard Quintin said investigators were working to determine how many drones were involved and who was behind the incident. The government is expected to convene its national security council on Thursday to address the disruptions.

“The recurrence of drone-related incidents directly affects the security of our country,” Quintin wrote on X. “We must act in a calm, serious and coordinated manner.”

The disruptions follow a string of drone incidents across Europe in recent weeks, including in Germany and Denmark, that have raised security concerns.

Over the weekend, Belgian officials also reported drone activity near the Kleine-Brogel military base, believed to house U.S. nuclear weapons.

A spokesman for air traffic controller Skeyes said flights resumed early Wednesday after the overnight suspension.

Ariane Goossens, spokeswoman for Brussels Airport, told AFP that between 400 and 500 passengers were stranded overnight but that “the situation will return to normal during the day” as operations resumed.

Liege Airport, a major freight hub that handles many overnight flights, also experienced a six-hour interruption before reopening. “The incidents were worrying for national security,” airport spokesman Christian Delcourt said.

Belgian media later reported additional drone sightings near the Kleine-Brogel base on Tuesday evening, though authorities have not confirmed them.

Defence Minister Theo Francken declined to directly link the activity to Russia but said the operation appeared to have been carried out by “professionals.”

“They are trying to sow panic in Belgium,” he told local media. “This is destabilisation.”

Military intelligence has opened an investigation to identify those responsible for the coordinated drone incursions.