Belarus grows more isolated as countries weigh punishment for the Ryanair flight force-down.

A day after it brazenly forced down a commercial airline so it could seize a dissident journalist traveling on board, Belarus found itself increasingly isolated on Monday, as other countries considered measures that would effectively make Belarusian airspace off limits to airlines.

“The reaction should be swift and be severe,” Belgium’s prime minister, Alexander de Croo, declared as European leaders prepared to gather in Brussels to discuss the next steps.

Condemnation grew over the diversion of the Ryanair flight, which had been ordered by the country’s strongman leader so that a Belarusian journalist traveling from Greece to Lithuania through Belarusian airspace could be detained.

President Biden was briefed Monday morning about the incident, which Secretary of State Antony J. Blinken condemned as a “shocking act” that “endangered the lives of more than 120 passengers, including U.S. citizens.”

He demanded the “immediate release” of the journalist, Roman Protasevich and a full investigation.

Flight-tracking data showed that airlines have already started to avoid the Eastern European country’s airspace, Reuters reported, but some European officials were calling for a formal ban.

Britain ordered that “airlines avoid Belarusian airspace in order to keep passengers safe,” the transportation secretary, Grant Shapps, wrote on Twitter.

Mr. Shapps also said that the operating permit for Belavia Belarusian Airlines was being suspended.

In Ukraine, Belarus’s neighbor to the south, President Volodymyr Zelensky directed his government to ban flights from Belarus and to close the Belarus airspace to flights to or from Ukraine.

And the Lithuanian government called for Belarusian airspace to be closed to international flights in response to what it called a hijacking “by military force.”

Michael O’Leary, the chief executive of Ryanair, an Irish-based low-cost carrier, called the operation, which was directed by President Aleksandr G. Lukashenko of Belarus, a “state-sponsored hijacking.”

Sofia Sapega, the girlfriend of the arrested journalist, was also detained when the plane landed in Minsk on Sunday after a bogus bomb threat during its flight from Athens to Vilnius, Lithuania, her university in the Lithuanian capital said.

Ms. Sapega, a Russian citizen, was detained at the Minsk airport along with Mr. Protasevich under “groundless and made-up conditions,” the European Humanities University in Vilnius said in a statement demanding her release.

There was no word Monday morning from the Belarusian authorities on their whereabouts.

Lawyers seeking to help Mr. Protasevich said he was believed to be in a jail in Minsk operated by the Belarusian intelligence service. The Russian Embassy in Minsk said that Belarus had notified it of Ms. Sapega’s detention.

Five people who boarded in Athens were not on the plane when it finally arrived in Vilnius, the Lithuanian police said on Monday.

Mr. O’Leary said some of the passengers may have been agents of the Belarusian intelligence service, which is still known by its Soviet-era initials.

“We believe there were some K.G.B. agents offloaded at the airport as well,” Mr. O’Leary told Irish radio on Monday.

Mr. O’Leary said Ryanair was in the process of debriefing its crew .

The Lithuanian police said they had opened a criminal investigation, on suspicion of hijacking and kidnapping. Of 126 passengers who took off from Athens, 121 arrived in Vilnius, the police said. (Officials had earlier said there were about 170 passengers on the plane, and that six had stayed behind in Minsk.)

The Lithuanian police spoke to the pilots after they landed in Vilnius on Sunday evening, Renatas Pozela, the Lithuanian police commissioner general, said in a telephone interview.

Police investigators would be interviewing the passengers this week, he said.

“The pilots were the priority,” Mr. Pozela said. “We wanted to hear their stories. How did they see the situation? What did they do? Were there other planes?”