Several drones spotted above a Belgian airbase over the weekend were there “to spy” on military fighter jets, the country’s defense minister said Sunday, at a time when European nations are on high alert for airspace intrusions.

At least 400 drone units are using the “Army of Drones Bonus System”, which killed or wounded 18,000 Russian soldiers in September, the newspaper reported. This was double the number from last October, after Ukraine’s government doubled the rewards for killing Russian infantry from six to 12 points.