The head of NATO has joined the leaders of several EU countries in demanding an investigation into the diversion on May 23 of a Lithuanian-bound flight to Minsk, where authorities arrested one of its passengers, opposition activist and journalist Raman Pratasevich.

NATO Secretary-General Jens Stoltenberg said that the alliance was closely monitoring the "forcible landing" of the flight -- from Athens to Vilnius -- in Belarus and the reported detention of Pratasevich.

"This is a serious & dangerous incident which requires international investigation. Belarus must ensure safe return of crew & all passengers," Stoltenberg said on Twitter.

Ryanair said the flight arrived safely in Vilnius on May 23 after a delay in Minsk of several hours. The Irish airline said earlier that "nothing untoward" had been found after it was notified of a potential security threat on board by Belarus air traffic control and instructed to divert.

Lithuanian President Gitanas Nauseda earlier slammed the arrest of Pratasevich on Twitter, calling it an “unprecedented event" and saying the Belarusian regime is “behind this abhorrent action.”

Lithuania later summoned the Belarusian ambassador and urged its EU allies to do likewise. It also called for EU countries to jointly recommend that planes avoid Belarusian airspace.

The proposal will be put forward at a meeting of European leaders on May 24 and will include a call to recognize the incident as a violation of International Civil Aviation Organization (ICAO) rules, the Lithuanian Foreign Ministry said.

"This is a brutal affront against all [the] EU," Foreign Minister Gabrielius Landsbergis said in a statement.

The ICAO said the incident "could be in contravention of the Chicago Convention," which prohibits any use of civil aviation that may endanger safety.

The ICAO is a UN agency directed by 193 governments, including Belarus, to support cooperation in air transport but has no regulatory power.

Polish Prime Minister Mateusz Morawiecki earlier asked the European Council's president to discuss immediate sanctions against Belarus during the May 24 meeting, and French Foreign Minister Jean-Yves Le Drian said the incident requires a "strong and united" response from the European Union.

The German and British foreign ministries also expressed alarm, and European Council President Charles Michel said an investigation by the ICAO "will be essential."

Belarusian state news agency BelTA reported that authoritarian leader Alyaksandr Lukashenka had personally ordered a fighter jet to escort the Ryanair jet, which was carrying more than 100 passengers, to land in Minsk.

Pratasevich was taken away by police shortly after the Ryanair flight landed in the Belarusian capital.

Pratasevich was a key administrator of the Telegram channel NEXTA Live, which has been covering the protests that broke out in Belarus following the country’s disputed presidential election last August.

Belarusian authorities in November launched investigations into Pratasevich and a colleague, Stsyapan Putsila, on suspicion of the organization of mass disorder, disruption of social order, and inciting social hatred.

Pratasevich was a 2017-18 Vaclav Havel Journalism fellow in Prague. The Vaclav Havel Journalism Fellowship -- a joint initiative of RFE/RL and the Czech Foreign Ministry -- is available to aspiring, independent journalists in the European Union's Eastern Partnership countries and Russia.

Pratasevich spoke to Current Time from an undisclosed location in Poland on November 19 after Belarusian authorities issued a warrant for his arrest.

"It seems to me that the [state] power now considers nearly any expression of a different opinion in general to be a crime,” Pratasevich said, saying this was clear from the number of people who were being detained. Current Time is a Russian-language network run by RFE/RL in cooperation with VOA.

Belarus has been rocked by protests since Lukashenka, in power since 1994, was declared the landslide winner of the poll amid allegations of vote-rigging. Since then, more than 30,000 people have been detained, hundreds beaten or tortured, and journalists targeted in the crackdown by Lukashenka, whose government has been hit by Western sanctions.

In October, a court in Minsk designated the NEXTA Live channel and its logo as extremist and instructed the Information Ministry to restrict access to information resources using the name and logo of the Telegram channel, as well as their distribution in the Belarusian segment of the Internet.

NEXTA Live then changed its name and logo, switching from the Latin transliteration of its name to a Cyrillic one.

Fearing prosecution, Pratasevich and Putsila fled the country and their whereabouts have not been known.

In October, Putsila, along with several Belarusian activists, received the European Parliament's 2020 Sakharov Prize for Freedom of Thought.

Media in Belarus have been targeted by the Lukashenka government in the ongoing crackdown. The watchdog Reporters Without Borders has designated Belarus as the most dangerous spot in Europe for journalists.

On May 21, Belarusian security forces raided a Minsk studio used by a Polish-based TV station that has produced investigations critical of Lukashenka and his associates.

Belsat said uniformed officers broke into a studio on May 21 used for producing a talk show, detaining six people, including four cameramen.

In April, the channel published an investigation into the business dealings of Lukashenka's daughter-in-law and others associated with him.

Earlier this year, two journalists for Belsat were handed what their lawyers called an "absurd" sentence of two years in prison each for reporting live from a rally in Minsk in November.

Earlier this week, police launched a probe of the country's largest independent online media outlet, Tut.by, searching the homes of several of its editors and blocking its website.

Meanwhile, a Minsk court on May 21 sentenced another reporter who covered the police raid on Tut.by to a 15-day prison sentence, a media advocacy group said.

The Belarusian Association of Journalists said 27 media workers are currently behind bars, either awaiting trial or serving sentences.

With reporting by AP, Reuters, AFP, and Current Time