ISTANBUL 

European Commission President Ursula von der Leyen warned on Tuesday of a "pattern of persistent contestation" at Europe's borders following drone incursions around Copenhagen Airport.

"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," von der Leyen wrote on US social media company X.

She also vowed to respond to the "threat" with "strength" and "determination."

European Council President Antonio Costa also voiced concern over the unidentified drone incursion into Danish airspace and expressed "full solidarity."

"The EU must be able to protect its critical infrastructures," Costa added.

Copenhagen and Oslo airports were forced to temporarily close on Monday evening after multiple sightings of unidentified drones in their airspace, authorities said.

Copenhagen police said “three or four large drones” were observed near the Danish capital’s airport around 8.30 pm local time (1930GMT).

In Norway, Oslo Airport also halted operations for several hours after two separate drone sightings.

Danish police official Jakob Hansen said security forces, military, and intelligence services were working jointly with Norwegian authorities to determine the origin of the drones.

The drone sightings follow a series of recent airspace violations in Europe.

On Sept. 10, a suspected Russian drone strike reportedly hit Polish territory, prompting NATO to deploy fighter jets. Similar accusations were made by Romania and Estonia.