Western politicians accused Belarus on Monday of state piracy amounting to a "warlike act", searching for a way to retaliate that would match the gravity of the offence after Minsk forced an airliner down and arrested a dissident journalist.
Sunday's action, in which a Belarusian warplane intercepted a Ryanair flight between European Union members Greece and Lithuania and forced it to land in Minsk, has few precedents, and denunciations were worded in the strongest terms.
Belarus authorities arrested a passenger, 26-year-old Roman Protasevich, whose social media feed from exile has been one of the last remaining independent outlets for news about the country since a mass crackdown on dissent last year.
"This was effectively aviation piracy, state sponsored," said Ireland's Foreign Minister Simon Coveney.
Swedish Foreign Minister Ann Linde said: "It is dangerous, reckless and naturally, the EU is going to act."
The head of the foreign affairs committee in the British parliament, Tom Tugendhat, noting that the flight was between two members of both EU and the NATO military alliance, said: "If it's not an act of war, it's certainly a warlike act."
Belarus says it was acting in response to a bomb threat on the flight, although this turned out to be false. It said on Monday its ground controllers had given guidance to the flight but had not ordered it to land.
Russia accused the West of hypocrisy, noting a precedent: that in 2013 a flight from Moscow carrying Bolivian President Evo Morales had been diverted to Austria after reports fugitive U.S. intelligence leaker Edward Snowden might be on board.
The issue was set to dominate a scheduled summit of EU leaders, but options for an effective response may be limited. The EU and the United States already imposed several rounds of financial sanctions against Minsk last year, which had no effect on the behaviour of long-serving leader Alexander Lukashenko, a close Russian ally who withstood mass demonstrations against his rule after a disputed election.
At the very least, the incident will disrupt air traffic patterns in Europe, with a Latvian airline, airBaltic becoming the first on Monday to announce it would no longer fly over Belarusian air space.
Ryanair's boss Michael O'Leary, who referred to the incident as a state-sponsored hijacking, said he believed security agents had been on the flight and had disembarked in Minsk. That would mean the operation had effectively been coordinated with spies operating on the ground in Greece.
"The EU will consider the consequences of this action, including taking measures against those responsible," EU foreign policy chief Josep Borrell said in a statement on Monday. European Commission chief Ursula von der Leyen said "the outrageous and illegal behaviour of the regime in Belarus will have consequences".
Minsk has shrugged off previous sanctions since last year, which consist mainly of adding various officials to black lists which restrict their right to travel or do business in Europe and the United States. The EU was already working on a fourth round before the Ryanair incident.
Additional steps could now include suspending overflights of EU airlines over Belarus or banning the Belarusian airline Belavia from landing at EU airports. An EU official said suspending ground transit could also be discussed.
Protasevich's whereabouts were not made public. A university in Vilnius said one of its students, Sofia Sapega, 23, was travelling with him and had also been detained.
After Protasevich was arrested, flight 4978 was allowed to travel on to Vilnius, where weary passengers disembarked. One, who gave his name as Mantas, described the moment when the pilot had come on the intercom to tell passengers they were being diverted to Minsk, with no explanation. Protasevich immediately shot to his feet, knowing his time was up.
"Roman stood up, opened the luggage compartment, took luggage and was trying to split things," giving a laptop and phone to his female companion, Mantas told Reuters. Once the plane landed, police took Protasevich away.
"We saw from the window that Roman is standing alone, and one policeman with dog was trying to find something" in his luggage, Mantas said.
Another exhausted passenger, speaking to reporters without giving her name, said Protasevich looked "super scared".
"I looked directly into his eyes and he was very sad."