European Commission President Ursula von der Leyen said Tuesday that Europe is facing a “pattern” of threats at its borders, after drones disrupted air travel in Denmark and Norway overnight.

“Just spoke with PM Frederiksen regarding the drones incursion around Copenhagen airport,” von der Leyen said.

“While the facts are still being established, it is clear we are witnessing a pattern of persistent contestation at our borders. Our critical infrastructure is at risk. And Europe will respond to this threat with strength & determination,” she added.

Both Copenhagen and Oslo airports shut down late Monday night after drones were spotted in their airspace, forcing flights to be diverted or canceled and leaving tens of thousands of passengers stranded.

Earlier Tuesday, Danish Prime Minister Mette Frederiksen said she “cannot rule out in any way that it is Russia” behind Monday night’s drone incident.

“What we saw last night is the most serious attack on Danish critical infrastructure to date,” she told Danish media, adding that the “obvious” motive behind the attack was “to disrupt and create unrest. To cause concern. To see how far you can go and test the limits.”

In recent weeks, Russia deployed drones over Poland and three Russian warplanes spent 12 minutes violating Estonian airspace before NATO jets diverted them.