Germany is investigating drone flights, suspected to be Russian surveillance drones, flying over key nuclear infrastructure in the north of the country.

The drones, which have not yet been identified, have been spotted flying over nuclear power facilities in Brunsbüttel, a city north of Hamburg near the coast of the North Sea, according to German newspaper Bild.

The tabloid reported that the Flensburg public prosecutor’s office, which opened an investigation into the drone flights, said that they were looking for “agent activity for sabotage purposes in connection with repeated drone flights.”

The Schleswig-Holstein state criminal police agency suspects that the drones are Russian Orlan-10s, previously used by Russia for surveillance and espionage, Bild said.

The airspace over Germany’s nuclear infrastructure is a no-fly zone. However, internal German police documents have revealed the zone has been breached several times in the last month.

Newsweek contacted the Flensburg public prosecutor’s office and the Russian Ministry of Defense for more information.

Germany has taken increasing action against sabotage in recent months. On August 14, it temporarily closed off part of a military base in Cologne after an unknown suspect broke in and tampered with the water supply. There is still no indication as to who broke into the base and what their motives were.

Germany has remained one of Europe’s largest supporters of Ukraine in the ongoing war with Russia, recently giving the all-clear for Ukraine to use weapons and equipment provided by the key NATO member as part of its Kursk offensive.

Earlier this year, German officials announced a $542 million arms package for Ukraine, which included ammunition for an IRIS-T air-defense system, used to counter Russian air strikes.

Drones have been a massive part of the conflict in Ukraine on both sides, with Ukraine using them aggressively to target Russian infrastructure and to down aircraft like helicopters.

Russian oil supplies have been particularly vulnerable to drone attacks, with one strike on August 18 causing a three-day fire that crippled the Proletarsk state reserve fuel facility, and required more than 500 firefighters to control.