NATO has changed its incident monitoring protocol after a Soviet-era military drone crashed in Zagreb last week, Slovenian Defence Minister Matej Tonin said on Wednesday.

Addressing a press conference after an extraordinary meeting of NATO defence ministers in Brussels, Tonin said that it was now known that the unmanned aerial vehicle had passed through the air space of at least three allied countries and that the flight had been registered.

Although the drone was registered by radar, the countries in question ignored it, considering it a reflection on radar image, Tonin said as quoted by STA news agency.

Immediately after the incident in Zagreb, NATO has changed its protocol for such situations, and now NATO countries’ aircraft must respond even if radar operators think what they see is only a reflection, he said.

Tonin said that Slovenia had put its aviation on alert after the Zagreb incident.