Russia likely used shadow ships to launch drones over Europe that repeatedly disrupted civilian aviation, as it monitored military sites and tested the air defenses of NATO nations, according to a report published Thursday by the International Institute of Strategic Studies think tank.

The report, which was shared before publication with The Associated Press, plotted 144 suspected drone sightings across Europe, including in NATO members Germany, France, Belgium, the Netherlands, the U.K., and Denmark, between 2024 and 2026.

Those sightings peaked in late 2025, forcing the temporary closure of several European airports, including in Germany, Spain and Denmark.

Danish Prime Minister Mette Frederiksen described the incidents in her country as the “most serious attack on Danish critical infrastructure to date.”

The IISS said the Russian campaign was designed to fall below the threshold of triggering discussions for a collective NATO response and was a “strategic failure” for Europe that exposed how the continent’s air defenses are not fit to deal with the current threat.

Air Chief Marshal John Stringer, NATO’s deputy supreme allied commander in Europe, told AP that it is up to each alliance member to decide how to respond to such threats — and many countries are taking them increasingly seriously.

Several senior European officials said it’s difficult to attribute the drone incursions to Russia, and Stringer did not blame Moscow. But he suggested the activity fits the pattern of behavior seen in a widespread campaign of disruption across Europe that Western officials have blamed on Russia since its invasion of Ukraine in 2022.

Of the handful of countries that have confirmed drone sightings, only Sweden has blamed Moscow directly — when a military drone flew toward a French aircraft carrier from a Russian spy ship.

President Vladimir Putin said in May that Russia is not waging a sabotage campaign against Europe.

European nations struggled to respond to drones, the IISS said

Drones are challenging to detect because they fly low and slow, and on radar can look like birds or planes. They can be launched inside, or near, national borders, bypassing missile defenses, which are designed to detect supersonic missiles fired from abroad. And even if a drone is shot down, it’s extremely difficult to identify where it came from or who sent it and why.

The incursions showed drone defenses in Europe need to rapidly be improved, the IISS said.

Creating a comprehensive picture of threats is often difficult because the responsibility for tackling drones is often split among various authorities, Lt. Gen. Jonny Lindfors, Sweden’s military representative to NATO, told AP. It’s also a “tough decision,” he said, to decide to shoot a drone down because it could cause civilian casualties.

In a June report, Denmark’s Defense Command said the armed forces “could have been in a stronger position” to respond to drone sightings and that there were shortcomings in equipment used to detect them. Following the incidents, Denmark and several other European countries said they would boost their drone defenses.

Countries that may have had a “benign” view of the danger a few years ago have now realized that counterdrone systems are a critical part of air defense, said Stringer.

Russia is launching drones from shadow ships, the IISS said

The IISS plotted the movement of Russian shadow fleet vessels — ships of uncertain ownership that help Moscow dodge sanctions — and concluded that it’s highly likely that Russia is using them as platforms for launching drones.

The report focused on sightings of drones across central and northwest Europe and did not examine incursions along NATO’s eastern flank, where multiple Russian and Ukrainian attack drones have entered European airspace.

On Dec. 2, the Vezhen, a shadow ship linked to the severing of a Baltic cable, sailed in circles off the coast of Ireland when Ukrainian President Volodymyr Zelenskyy arrived for his first official visit, the IISS said. Another ship was nearby, but its signal was off, it said.

The report cited other sightings that occurred when shadow fleet ships were nearby, including several in Denmark.