BRUSSELS (AP) — Dozens of flights at Belgium’s main international airport were canceled on Wednesday after drone sightings overnight forced it to close temporarily, prompting Prime Minister Bart De Wever to convene a meeting with senior ministers to discuss safety concerns.

It was the first time that the airport in Brussels has been shut down by drones. It comes after a series of unidentified drone flights over the weekend near a military base where U.S. nuclear weapons are stored.

Brussels Airport said that 54 flights were canceled as a result of the Tuesday evening shutdown “for safety reasons” and apologized, saying that “the safety of our passengers and staff remains our top priority.”

The airport operator said that about 400-500 travelers spent the night there after 41 flights were canceled and 24 diverted elsewhere, and that it had set up beds and distributed water and snacks to keep people comfortable.

De Wever convened a meeting of Belgium’s National Security Council, which includes the country’s defense, interior, justice and foreign ministers, for Thursday morning.

In a post on social media, Interior Minister Bernard Quintin said that “the repetition of incidents linked to drones directly affects the security of our country. … We must take action in a calm, serious and coordinated manner.”

The operators of the drones in recent days have not been identified, but Defense Minister Theo Francken insisted that "this is not the work of amateurs," without elaborating.

Earlier this week, Francken said that flights near the Kleine-Brogel air base on Saturday and Sunday nights appeared to be “a spying operation” aimed at “destabilizing” people.

He said that drones had been flown near the base in “two phases,” one involving small drones that tested the radio frequencies being used by Belgian security services to jam such flights; the second with bigger drones that operated on a different frequency to avoid the jammers.

Last month, several drones were spotted above another Belgian military base near the German border. The operators were not identified.

In recent months, drone incidents across Europe have forced airports to shut down for a time. A late evening drone sighting at Berlin’s Brandenburg airport on Friday suspended flights for nearly two hours. It was not clear who was responsible.

Belgium hosts the headquarters of NATO and the European Union, as well as Europe's biggest financial clearinghouse holding tens of billions of euros in frozen Russian assets. Many EU countries want to use those assets as collateral to provide loans to Ukraine, but Belgium has so far resisted.

De Wever warned EU leaders last month that Russia might retaliate if the money is confiscated.