Airports in Norway and Denmark faced hours of disruption on Monday night after drone sightings caused flight delays and cancellations.

Copenhagen airport – the busiest in the Nordic region – reopened early on Tuesday after drone sightings halted take-offs and landings for nearly four hours, the airport said.

“The police have launched an intensive investigation to determine what kind of drones these are,” Copenhagen police deputy assistant commissioner Jakob Hansen said. “The drones have disappeared and we have not taken any of them.”

The airport said delays and some cancelled departures would persist and passengers should check with their airlines.

Oslo airport shut its airspace from midnight local time due to a drone observation, a spokesperson for Norwegian airport operator Avinor told Reuters, adding that all flights were diverted to the nearest airport.

Hansen said authorities in Denmark and Norway would cooperate to determine whether there was a link between the two incidents.

Earlier, Danish police said that two or three large drones had been seen flying in the area around Copenhagen airport, the country’s biggest. About 50 flights were diverted to alternate airports, according to flight-tracking website FlightRadar.

An airport spokesperson said police were working to identify the drones but declined to provide further details, citing the ongoing investigation.

With Reuters