Drone sightings forced the authorities in Denmark and Norway to close the main airports in Copenhagen and Oslo for several hours overnight, causing widespread flight disruptions into Tuesday. It was not immediately clear where the drones originated, who was operating them, or whether the Oslo and Copenhagen sightings were linked. But Prime Minister Mette Frederiksen of Denmark said in a statement that it was the “most serious attack so far” on the country’s critical infrastructure and that her government was not ruling out any possibilities. Several European countries, especially those near Ukraine, went on high alert for incursions after Russian aircraft entered the airspace of Poland, Estonia and Romania in recent weeks. Ms. Frederiksen noted that the Copenhagen police had said the episode involved a “capable actor,” and said it was “clear that this ties into the developments we have observed recently with other drone attacks, violations of airspace, and cyberattacks on European airports.”