Suspicious drones spotted above Belgian military bases over the weekend are meant to spy on fighter jets and ammunition, Belgian Defense Minister Theo Francken said Sunday.

“They [the drones] come to spy, to see where the F-16s are, where the ammunition are, and other highly strategic information,” he told Belgian outlet RTBF.

Francken earlier Sunday announced that an investigation was launched after reports of unmanned aerial vehicles flying over the Kleine Brogel military base in northern Belgium.

The base is key to Belgium’s defense and includes an American presence. That’s where Belgium’s U.S.-made nuclear-capable F-16 fighter jets are currently stationed, and where F-35s will be located as well.

This weekend’s incident is one of many recently in Belgium and elsewhere in Europe. In the past weeks, Russian drones were intercepted and shot down over Poland. Another Russian drone was tracked over Romania and drones of unknown origin — but with high suspicion toward Russia — disrupted air traffic at airports in Denmark, Norway and Germany.

Francken stopped short of attributing the latest drone incursions in Belgium to Moscow, but he hinted there were few other obvious culprits. “The Russians are trying to do this in all European countries,” he told RTBF. “Is it the Russians now? I can’t say that, but the motives are clear and the ways of doing things like this are also very clear,” he said.

“War is truly a drone war, and the Defense Department really needs to prepare for that,” Francken added.

The Belgian defense minister is expected to present next week a €50 million plan to deploy a national counter-drone system. Speaking on Radio 1, he previously said the package will fund detection systems, jammers and drone guns to protect key installations.