Large drones that flew over Copenhagen airport for hours and caused it to shut down constituted the “most serious attack on Danish critical infrastructure” to date, Prime Minister Mette Frederiksen said Tuesday.

Airports in Copenhagen and Oslo reopened early Tuesday, hours after unidentified drones in their airspace caused dozens of flights to be diverted or cancelled, disrupting travel for thousands of passengers.

“This is part of the development we have recently observed with other drone attacks, airspace violations, and cyberattacks targeting European airports,” Frederiksen said in a statement sent to AFP.

She referred to similar drone incidents in Poland and Romania and the violation by Russian fighter jets of Estonia’s airspace.

The governments of Poland, Estonia and Romania have

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