Michael O'Leary says the forced diversion of a Ryanair flight to Belarus was a "state-sponsored hijacking".
The airline's CEO says it appears authorities removed a journalist and his travelling companion from the plane, while the company also believes there were some KGB agents on board the flight.
The plane - which was flying from Greece to Lithuania - was escorted by a Belarusian fighter jet after reports it had explosives on board, but none were found.
It was forced to divert from Minsk, where an opposition blogger - Roman Protasevich - was arrested.
European leaders will discuss a response to the incident today, after several officials called it "utterly unacceptable".
Foreign Affairs Minister Simon Coveney has described it as "aviation piracy that's state-sponsored".
It is utterly unacceptable. This is an Irish Airline with EU citizens on board, forced to land in Minsk, while travelling between EU cities. A strong an united response from EU needed.
EU inaction or indecision will be taken as weakness by Belarus. @dfatirl https://t.co/WUwdcsTjSU
— Simon Coveney (@simoncoveney) May 23, 2021
On Newstalk Breakfast, Mr O'Leary said he thinks it's the first time such an incident has happened to a European airline.
He said: "This was a case of state-sponsored hijacking… state-sponsored piracy.
“I can’t say much about it because the EU authorities and NATO are dealing with it at the moment.
“We’re debriefing our crew, who did a phenomenal job to get that aircraft and almost all the passengers out of Minsk after six hours."
Mr O'Leary said the incident was "very frightening" for passengers and crews, as they were held under armed guard and had their bags searched.
He said: "It appears the intent of the authorities was to remove a journalist and his travelling companion… we believe there were some KGB agents offloaded at the airport as well."
There are growing calls from European politicians - including in Ireland - for flights to be banned from flying over Belarus.
Mr O'Leary said his airline will take guidance from European authorities about whether that should happen.
However, he said they don't fly over Belarus much - and it would be a "very minor adjustment" to fly over Poland instead.
Our updated statement regarding yesterday's (23 May) diversion of FR4978. pic.twitter.com/BANHrDvXMq
— Ryanair Press Office (@RyanairPress) May 24, 2021
Minister Coveney, meanwhile, says the plane was "effectively ordered" to land in Minsk.
He said: "Five or six people left the plane, but only one of them was arrested - which would suggest the others were secret service people.
"This is effectively a state kidnapping by a regime run by a dictator."
He said there are lots of people who are opponents of Belarusian President Alexander Lukashenko - a leader who Minister Coveney said has "no democratic legitimacy".
He said: "There's a European Council meeting starting this evening... I think you will see decisions taken by the EU to increase the level of sanctions against Belarus, and perhaps to link that to aviation.
"I think if the EU doesn't respond firmly and directly, then I think that will be seen by Belarus as weakness on behalf of the EU."
Meanwhile the Russian ambassador to Ireland has said there should not be "any rush to judgement" over the incident.
Ambassador Yury Filatov told The Hard Shoulder a full investigation needs to be carried out.
"There are many tough words around now - I don't think there should be any rush to judgement.
"There are clear rules which govern the airspace, the air travel under the International Civil Aviation Organisation and Convention.
"And I think that the incident has to be thoroughly investigated by the international aviation authorities".