The U.S. has an estimated 200 tactical nuclear weapons, with roughly half deployed in Europe. The U.S. is believed to have around 100 tactical bombs deployed in five NATO countries on the continent, including in Turkey, Germany and Belgium.

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.