Air traffic at Stockholm's Arlanda international airport was halted for two hours overnight after four drones entered its air space, police said Monday, adding that they suspected sabotage.

Police alerted air traffic controllers to the drones between 1:00 am and 2:00 am (2300 and 0000 GMT), Sweden's Civil Aviation Administration told AFP.

"We concluded that we were unable to guarantee aviation security and decided to restrict the air space," agency spokesman Daniel Ackermann said.

Take-offs and landings were halted between 2:00 am and 3:40 am, affecting five flights, airport operator Swedavia told AFP.

The five were either re-routed to other airports or delayed.

Police have opened an investigation into "airport sabotage", they said in a statement.

"We suspect that this is a deliberate act, but I can't say what the motive may have been," police spokesman Daniel Wikdahl told several Swedish media.

"Because of confidentiality due to the ongoing investigation, we can't say what kind of drones were used," he added.