Police shut down Copenhagen Airport on Monday night after spotting two to three suspected Russian drones, airport officials said.

Authorities reported that flights resumed a few hours later, though over 30 were diverted and delays persisted into Tuesday.

Danish Prime Minister Mette Frederiksen on Tuesday described drone incursions as “the most serious attack on Danish critical infrastructure to date.” 

“We are obviously not ruling out any options in relation to who is behind it. And it is clear that this fits in with the developments we have observed recently with other drone attacks, violations of airspace, and hacker attacks on European airports,” Frederiksen added.

Norway’s Oslo Airport reported a separate drone sighting the same evening, forcing four hours of airspace restrictions before reopening Tuesday morning.

The two incidents follow a series of drone incursions across Europe, including in Romania, Poland and Estonia. 

Russia, which has repeatedly denied involvement in recent drone incursions, also denied any role in the latest airport drone incidents.

“Frankly, making unfounded accusations over and over again only leads to such statements no longer being taken seriously,” Kremlin spokesman Dmitry Peskov told reporters during a daily press briefing. “Every time, we hear the same baseless claims. A side that claims to take a serious and responsible position should not be making such accusations time after time.” 

On Monday, Poland’s Prime Minister Donald Tusk warned that the country would shoot down any Russian aircraft or drones that enter its territory.

The recent airspace violations prompted NATO Secretary-General Mark Rutte to launch the “Eastern Sentry” mission on September 12, aimed at strengthening alliance defenses along the eastern flank.