Denmark’s Prime Minister Mette Frederiksen warned Danes to brace for more hybrid attacks, pointing to Russia as Europe’s top security threat as suspected drone sightings occurred for the third time in a week.

Authorities still cannot conclude who is behind several drone incidents across Danish airports in recent days, but Russia remains Europe’s main adversary, seeking to destabilize the continent, Frederiksen said in a video speech on late Thursday.

“These are attacks we must expect more of. These are attacks that have exposed vulnerabilities,” she said, adding that Danes should also prepare for more sabotage, cyberattacks and the destruction of subsea cables.

The warning came just hours before authorities closed the airspace over Aalborg airport and military air base over suspicion of illegal drone activity. Two flights were canceled, broadcaster DR reported. The airspace was closed for about an hour, reopening just after midnight on Friday. No drone sighting was confirmed, demonstrating how on edge authorities are.

The incidents come ahead of a high-level meeting of European leaders in Copenhagen next week to discuss security and defense, including how to deal with recent hybrid attacks and Russian airspace incursions.

Denmark is attending a meeting on Friday with about half a dozen European Union countries about the creation of a “drone wall” to protect the bloc’s eastern flank, and has called for a session of parliament’s foreign affairs committee to address the incidents.

Reports of illegal drone activity also led authorities to briefly close the airspace over Billund airport on Friday morning, though no flights were affected, TV2 reported, citing police. It later turned out to be a bright star in the sky.

Denmark is one of Ukraine’s strongest supporters, having donated large quantities of military equipment and financial support, and now allowing Ukrainian defense companies to set up operations on its soil. Earlier this month, the government also announced it would buy long-range missile systems capable of striking targets in Russia. Some analysts suggest those moves may have triggered Russian response.

“If it is Russia, it would fit a pattern where some of the countries most active in criticizing Russia and supporting Ukraine are subjected to hybrid threats,” said Sten Rynning, professor of war studies at University of Southern Denmark. Authorities are still investigating who were behind the Danish incidents, but said they were likely carried out by a professional agent, possibly a state actor.

The risk of Russian espionage and sabotage in Denmark is “high,” Finn Borch, director of the Danish Security and Intelligence Service, said at a news conference on Thursday evening, adding that the events resemble hybrid warfare seen elsewhere in Europe. Denmark’s investigation into the episodes includes studying the movement of ships.

Russia has denied involvement in the drone incidents.

Danish authorities have raised the alert level and are preparing for different scenarios, Frederiksen said. It means the military and police will be more visible with anti-drone capacity around critical infrastructure.

The government has come under intense criticism for allowing unauthorized drones to fly undisturbed over Denmark’s most critical infrastructure for hours, facing questions over why authorities failed to detect them earlier and why no measures were taken to intercept or bring them down.

The premier said she’d been in close contact with several European leaders in recent days, including counterparts in Germany, the U.K., and the Nordic countries and the North Atlantic Treaty Organization about how they can support Denmark.

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—With assistance from Julia Janicki.

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