The largest airports in Denmark and Norway were brought to a standstill on Monday night and Tuesday morning after drones were spotted overhead.

The sightings followed days of chaos at several other European airports attributed to a suspected cyberattack.

Police in Denmark said on Tuesday it was unclear how many drones had been seen above Copenhagen Airport but that they appeared to have been piloted by a "capable operator" who may have simply wanted to "show off".

They also said they did not know whether the drones over Copenhagen Airport were linked to drones spotted a few hours later over Norway's Oslo Gardermoen Airport, or whether any nation was behind the incidents.

Both countries scrambled their armed forces in response to the sightings, which happened at around 8:30 pm, local time, in Copenhagen and at about midnight in Oslo. Both of the airports reopened around four hours after they closed, although delays and disruption continued for several more hours.

A police spokesperson said "a number of measures will be implemented" to reduce the chance of something similar happening again.

Copenhagen Airport said in a statement it believes two or three large drones entered its airspace.

Norwegian broadcaster NRK said the drones over Oslo Gardermoen Airport arrived from multiple directions and seemed to disappear after they had caused disruption.

Copenhagen's Deputy Police Inspector Jakob Hansen said he did not know anything about the origin of the drones seen on Monday evening. He also confirmed none had been intercepted.

Hansen added that there was nothing to suggest the drone operator intended to harm anyone.

A spokesperson for Copenhagen Airport said as the incident was unfolding "no aircraft can take off or land at the airport, and as a result, several flights are being diverted to other airports".

The incidents followed apparent cyberattacks that began on Friday and caused disruption to several European airports' check-in and boarding systems by targeting software they use called Muse.

The European Union Agency for Cybersecurity, or ENISA, said on Tuesday it believes the cyberattacks were caused by criminals who used ransomware to create chaos and who then demanded bitcoin payments to reverse the damage they caused.

ENISA told the Reuters news agency: "The type of ransomware has been identified. Law enforcement is involved to investigate."

The BBC said many European airports, including Heathrow Airport, the UK's main flight hub, switched to backup systems after the attacks. It added that around half of the flights out of Heathrow were operating again by Sunday, although delays continued.

Berlin Brandenburg Airport, Dublin Airport, and Brussels-Zaventem Airport were among other major transportation hubs that were seriously impacted by the cyberattacks.