Belgium’s defense minister voiced alarm Monday over a series of unidentified drone flights near a military base believed to house U.S. nuclear weapons, suggesting the activity may be linked to espionage.

Defense Minister Theo Francken said the drones were detected over two nights, Saturday and Sunday, near the Kleine Brogel air base in northeastern Belgium. The first wave involved smaller drones “testing the radio frequencies” of Belgian security services, followed later by larger ones “to destabilize the area and people,” he told public broadcaster RTBF.

“It resembles a spy operation. By whom, I don’t know,” Francken said, noting he had “a few ideas” but declined to speculate publicly.

The incidents follow similar unexplained drone sightings over another Belgian military base near the German border last month. Recent months have also seen drone-related airspace violations across Europe, with Russia suspected in cases involving Estonia and Poland, though flights over Denmark and Germany remain unsolved.

Last Friday, flights at Berlin’s Brandenburg Airport were suspended for nearly two hours due to a drone sighting.

Francken dismissed the possibility that the Belgian incidents were a prank, saying the drones’ operators demonstrated advanced knowledge by evading jamming systems. “An amateur doesn’t know how to do that,” he said.

He added that shooting down drones near populated areas poses legal and safety risks: “When they’re over a military base, we can shoot them down. But nearby, they could fall on a house, a car, or a person.”

Francken admitted Belgium is “chasing after the threat,” adding that the country should have invested in modern air defense systems “five or 10 years ago” to better counter drone incursions.