The aircraft flew over Hungary, crossed briefly into Serbia and then Romania before entering Bulgarian airspace at 1609 GMT on Wednesday, the Bulgarian defence ministry said in a statement.

The twin-engine Beechcraft, with two people onboard, had no approved flight plan and its transponders were turned off. The pilot did not respond to radio requests and visual signals, the Romanian defence ministry said.

Two Hungarian Gripen fighter jets, two US F-16s and two Romanian F-16s intercepted the intruder one after the other and escorted it until it entered Bulgarian airspace, Bucharest added.

Bulgaria’s air force did not immediately send up fighters as “the plane was not considered a threat at any moment”, Defence Minister Dragomir Zakov told journalists.