Who caused planes to vanish on radars?
Dozens of aircraft have briefly vanished from air traffic control radar screens in Austria, Germany, the Czech Republic and Slovakia.
NATO has said it was not to blame for these recent incidents.
The Slovak state Air Traffic Services company said that the brief disappearance of planes from radar screens on June 5 and 10 was connected to a military exercise whose goal was "the interruption of radio communication frequencies."
It did not identify the military force organising the exercise, but Austrian media said it was NATO.
The military alliance, in response to a request for comment, said it did carry out some training that involved "localised and low-power jamming" in the skies over Hungary during the June 2-6 period and that it was currently conducting similar training in southern Italy from June 9-20.
However, it said no jamming was conducted on June 5.
"Our assessment is that NATO did not cause any interference with civilian air traffic control frequencies. When NATO conducts such exercises, we co-ordinate our activities with relevant civilian authorities and only use frequencies provided to us by the host nation," a NATO military officer said.
NATO AWACS surveillance aircraft that have been flying regular missions over Europe to monitor Russian activities near Ukraine could not have caused the interference because they did not have a jamming capability, the officer said.
The European air navigation safety organisation EUROCONTROL and the European Air Safety Agency are investigating the incidents.
"We will co-operate with these authorities in order to be absolutely sure that there is no connection between our exercises and the frequency interference issues that have been reported," the NATO officer said.