Dozens of aircraft briefly vanished from air-traffic control radars in Austria, Germany, the Czech Republic and Slovakia over the past two weeks in incidents Slovak authorities blamed on military electronic warfare exercises.
Air-traffic controllers in Austria and Germany said data about the planes' position, direction, height or speed went missing on June 5 and June 10, but the outages posed no serious danger.
Czech and Slovak controllers also encountered vanishing aircraft on the same days.
"The disappearance of objects on radar screens was connected with a planned military exercise which took place in various parts of Europe on June 5 and 10 and whose goal was the interruption of radiocommunication frequencies," the Slovak state Air Traffic Services company said.
"Immediately after the identification of the problem with the displays, the side organising the exercises was contacted and the exercise was stopped."
It did not identify the military involved, but Austrian media said it was Nato.
The incidents follow the disappearance of Malaysian Airlines flight MH370 in March.