Meeting in Brussels on Monday, the 27 national leaders of the bloc demanded an immediate release of dissident journalist Roman Protasevich, as well as an investigation by the International Organisation for Civilian Aviation into a Sunday incident during which Belarus forced a Ryanair flight to land in Minsk.
"We are closing our airspace to planes from Belarus and call on EU airlines not to fly over the country," said the head of the bloc's executive, European Commission President Ursula von der Leyen. "Further economic sanctions will be presented soon."
The EU currently has a travel ban and an asset freeze in place on 88 Belarusians, including Alexander Lukashenko, and seven companies, over Minsk's crackdown on protests following a contested presidential election last year.
Further individual sanctions could target oligarchs bank-rolling Lukashenko, diplomats said.
In a video posted online, the detained blogger Roman Protasevich (26) said he was in good health, being held in a pretrial detention facility in Minsk, and acknowledged having played a role in organising mass disturbances in the capital last year.
In the video on the Telegram messaging app, he wore a dark sweatshirt and clasped his hands tightly in front of him. The comments were immediately dismissed by his allies as having been made under duress.
A Polish deputy foreign minister, Pawel Jablonski, told private broadcaster TVN24 that his government had heard from Protasevich's mother about his being in poor health but provided no details.
Belarus's interior ministry said Protasevich was being held in jail and had not complained of ill-health.
Protasevich's social media feed from exile has been one of the last remaining independent outlets for news about Belarus since a mass crackdown on dissent last year. Sophia Sapega, a 23-year-old student travelling with him, was also detained.
OPTIONS APPEAR LIMITED
Some airlines and countries did not wait for guidance on how to respond to the diversion of the Ryanair flight.
Britain said it was instructing British airlines to cease flights over Belarus and that it would suspend the air permit for Belarus's national carrier Belavia with immediate effect. KLM, the Dutch arm of carrier Air France KLM, will temporarily halt flights, Dutch news agency ANP reported.
Still, the options for Western retaliation appear limited.
The Montreal-based ICAO has no regulatory power, and the EU has no authority over flights taking off and landing in Belarus or flying over its airspace, apart from direct flights that originate or land in Europe.
Belarus lies on the flight path of routes within Europe and between Europe and Asia, and skirting Belarus would slow flights down and cost airlines money.
The EU and the United States imposed several rounds of financial sanctions against Minsk last year, which had no effect on the behaviour of authoritarian President Alexander Lukashenko, who withstood mass demonstrations against his rule after a disputed election.
Lukashenko denies election fraud. Since the disputed vote, authorities rounded up thousands of his opponents, with all major opposition figures now in jail or exile.
'BOMB THREAT'
NEXTA, a news service where Protasevich worked before setting up his own widely followed blog, ran an interview with his mother, who said that as soon as she heard reports of a bomb scare on a flight, she knew it was a plot to capture him.
"I just want to say that my son is simply a hero, simply a hero," Natalia Protasevich said, weeping. "I truly hope that the international community will wake up for him."
Belarus says it acted in response to a false bomb threat written in the name of the Palestinian militant group Hamas. Hamas spokesman Fawzi Barhoum denied his group had any knowledge or connection to the matter.
Belarus said its ground controllers had given guidance to the flight but had not ordered it to land. State media said the intervention was ordered personally by Lukashenko.
Ryanair boss Michael O'Leary, who referred to the incident as a state-sponsored hijacking, said he believed security agents had been on the flight.
Lithuanian authorities said five passengers never arrived, suggesting three others besides detainees Protasevich and Sapega had disembarked in Minsk.
Russia, which has provided security, diplomatic and financial backing to Lukashenko, accused the West of hypocrisy.
