Danish Prime Minister Mette Frederiksen warned Thursday a “hybrid war” has started in Europe after waves of drones shut down Danish airspace this week.

Danish authorities temporarily closed two major airports overnight Wednesday after drones were sighted in the skies, in what Copenhagen called a “hybrid attack” by a “professional actor,” with several other airports across the country reporting similar incidents. Copenhagen and Oslo airports were also shut down Monday due to drone incursions, forcing flights to be canceled and stranding thousands.

The airspace breaches showed “we are at the beginning of a hybrid war against Europe,” Frederiksen said in an address to the nation posted on social media. “I think we are going to see more of it … We see the pattern, and it does not look good,” she added.

Frederiksen said Danish authorities had yet to identify “who is behind the hybrid attacks against our airports and other critical infrastructure” but hinted the Kremlin was responsible.

“We can at least state that there is primarily one country that poses a threat to Europe’s security — and that is Russia,” she said.

Europe has been rattled by a series of airspace incursions involving Russian jets and drones in recent weeks. Estonia and Poland convened NATO members for urgent talks after accusing Russia of violating their airspace in separate incidents.

Moscow has denied responsibility for the Estonian incursion and said the Polish incident, which saw a swarm of drones cross into Polish airspace, was an accident.

Just hours after Frederiksen’s address, Aalborg Airport in the country’s north was briefly closed for the second night in a row due to a suspected drone incursion.

Frederiksen said she could not promise “no drones will cross the border” but added Copenhagen had “raised the alert level” and was stepping up its drone-repelling defenses.

“This means that the defense and police will be more present with anti-drone capabilities around critical infrastructure in the coming time,” she said, adding Ukraine was providing expertise on combatting drones.

Earlier this month, the Danish Defense Ministry announced it would purchase the Franco-Italian SAMP/T air defense system for 58 billion Danish kroner (€7.7 billion), its largest arms purchase ever.

Frederiksen ended her address by calling for Europe to ramp up defense spending.

“That is why we are expanding the European defense industry, and that is why we are building up the defense industry in Denmark,” she said. “The events of recent days emphasize how important this is.”