BERLIN (AP) — German authorities are investigating claims that unidentified drones may have spied on critical infrastructure in the northern state of Schleswig-Holstein, officials said Wednesday. The sightings come as many NATO countries, including Germany, are on high alert following a series of recent incidents. There has also been a spate of drone incidents in Denmark, which neighbors Schleswig-Holstein. Authorities in Germany have urged caution about reading too much into drone sightings. Later, a “combined drone formation” was observed over the university hospital and a power plant, and other drone sightings were reported over governmental buildings and the Heide oil refinery in the area, the report said.