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.

Multiple drones were spotted on Sept. 25 over a power plant in the state capital of Kiel, as well as near a university hospital and a shipyard in the port city, according to a Der Spiegel report.

Regional Interior Minister Sabine Sütterlin-Waack told a committee of the state parliament on Wednesday that flying objects of “various types and sizes” had been spotted. Chief Public Prosecutor Stephanie Gropp said an investigation was under way.

The sightings come as many NATO countries, including Germany, are on high alert following a series of recent incidents. Polish forces shot down Russian drones violating Polish airspace, while Estonian airspace was breached by a Russian fighter jet. There has also been a spate of drone incidents in Denmark, which neighbors Schleswig-Holstein.

Schleswig-Holstein has been strengthening its drone defenses in coordination with other states in northern Germany, Sütterlin-Waack was cited by the dpa news agency as saying.

Authorities in Germany have urged caution about reading too much into drone sightings.

“I will explicitly say that not every drone controlled by foreign powers is automatically a threat either,” Interior Minister Alexander Dobrindt said Monday. Some incidents could be seen as “provocation” — as was the case in the sightings last week in Schleswig Holstein, he said.

Dobrindt said authorities must “respond appropriately” to the respective threat.

Der Spiegel said two small drones had been seen over the Kiel factory premises of TKMS, a maritime defense technologies provider, late Thursday evening.

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.

Further suspicious drones were spotted Thursday over a military base in Sanitz, in the Mecklenburg-Western Pomerania state to the east, the weekly reported.

Officials at the Federal Interior and Defense ministries, as well as state authorities, contacted by The Associated Press did not immediately respond to requests for comment.

Federal police, contacted by the AP, deferred to prosecutors investigating the case.