It's a loophole big enough to fly an Airbus through.
With most passenger routes between Moscow and other European capitals grounded by airspace bans enacted since Russia's large-scale invasion of Ukraine, two Serbia-based airlines are still flying passengers in and out of the Russian capital.
State-owned Air Serbia and privately owned Air Pink have initially exploited Belgrade's refusal to join EU sanctions on such flights, potentially dulling the intended effect on pro-Kremlin elites and other well-heeled Russians even as global outrage mounts over the humanitarian cost of the conflict.
Prices for seats on Serbian commercial flights out of Moscow spiked this week, but the routes were selling out anyway.
The flight bans are part of an unprecedented response to Russia's invasion of Ukraine, including cutoffs from banking and capital markets, exclusion from major sports competitions, and sanctions targeting President Vladimir Putin and his circle.
"We should be very careful about the transportation of oligarchs, businessmen, and politicians who may be from the narrow circle of the governing structure in Moscow," Nikola Lunic, executive director of the Council for Strategic Policy in Belgrade, told RFE/RL's Balkan Service.
On February 27, the same day that European Commission President Ursula von der Leyen announced the EU-wide ban, an aircraft belonging to Air Pink flew to Belgrade from Russia's second-largest city, St. Petersburg, according to tracking site FlightRadar24. Other European nations that are not members of the EU, including the United Kingdom and Norway, have also closed their airspace to Russian aircraft.
The next day, Air Serbia and Air Pink each flew routes from Moscow to Belgrade Nikola Tesla Airport.
Belgrade airport officials referred RFE/RL to the airlines for information about passenger numbers, and neither airline responded to questions for this article.
Air Serbia flies eight times a week between Moscow and Belgrade, and also has a route to St. Petersburg. It did not respond when asked if it planned to increase the frequency or capacity of its Russian routes.
Charter carrier Air Pink has a fleet of 15 planes that hold a maximum of eight passengers plus one aircraft that holds up to 13 passengers. It is owned by Zeljko Mitrovic, a Serbian entrepreneur with heavy media holdings who is seen as close to the current government.
Last week, FlightRadar24 showed Air Pink planes touching down in Moscow and St. Petersburg, Nice, Thessaloniki, and Palma de Mallorca, among other places.
The airspace bans have blocked Russia's Aeroflot and Nordwind airlines access to most European airports, including Belgrade, which is surrounded by countries that now prohibit Russian overflights.
Serbia isn't a member of the European Union, and President Aleksandar Vucic has spent years pursuing a "four-pillar" foreign policy that seeks to combine greater European integration with closer ties to Moscow and Beijing.
Vucic has aggressively courted Moscow for diplomatic support to block recognition of former province Kosovo, to supply military equipment, and to provide cheap gas and investment.
He and his ruling Serbian Progressive Party are facing a trio of crucial electoral tests next month, and polling suggests most Serbians regard Russia as a "friendly" state.
Last week, Vucic called Putin's invasion a "grave mistake" and a violation of Ukraine's territorial integrity, but he rejected calls for Belgrade to join sanctions on Russia.
Brussels is clearly watching closely.
"We have to remain vigilant on the impact of the crisis on the Western Balkans and look what's happening on the position, on the alignment of the countries of the Balkans, of our candidates to the European Union, their alignment with our foreign policy," EU foreign policy chief Josep Borrell said last week.
European Commission spokeswoman Ana Pisonero has since added that "candidate countries, such as Serbia, are expected to comply with the EU's decision and position on foreign policy and security issues."
Asked specifically about Serbian policies at a briefing on the EU airspace sanctions on March 1, a senior EU official who requested anonymity acknowledged the issue and said "we are organizing an outreach to neighboring countries and other third countries to insist on the importance of adopting these types of measures so far."
The official stressed that Russian companies are prevented from selling tickets through Belgrade and on to EU destinations but said the EU hadn't attached "extraterritoriality" to the current measures.
"[So] unless Serbia or other countries align, Air Serbia would be able to sell those tickets," the EU official said. "That's the situation today."
But the source insisted the bans "will not allow an oligarch to rent a plane to fly via Serbia because the sanctions cover also all non-Russian registered aircrafts that are either controlled, owned, or chartered by a Russian citizen."
Russian oligarchs and Putin cronies have been accused of stealing untold billions during the two decades of his rule.
As asset freezes and other punitive measures were being put into place over the past week, the Russian superrich appeared to be seeking safe harbor for things like superyachts.
U.S. President Joe Biden announced in his State of the Union address on March 1 that the United States would immediately be closing its airspace to Russian aircraft, and Canada has done the same.
Even Switzerland softened its longstanding policy of "neutrality" to join EU sanctions on Russia.
Limiting Russians' Options
The Serbian state owns 82 percent of Air Serbia since a recapitalization one year ago diluted the stake of Emirati strategic partner Etihad Airways.
Experts on air transport meanwhile doubt that there are significant long-term gains for a regional operator like Air Serbia, with its limited fleet of one wide-body, 12 narrow-body, and five turboprop planes.
"I think the amount of capacity at stake is very nominal and additionally, difficult to access geographically," air transport consultant John Strickland, director of JLS Consulting, told RFE/RL.
He and other experts said bigger outside operators like Turkish Airways, with its Istanbul hub, probably stood to profit more from the Western airspace bans and Russia's counterbans.
Russia responded to the flight bans on February 28 by closing its airspace to airlines from 36 countries, including all 27 members of the EU, the United Kingdom, and Canada.
Qatar Airways and even Emirates, a fellow Emirati competitor to Air Serbia minority stakeholder Etihad Airways, could profit, according to Petar Vojinovic, editor of the Serbian aviation portal Tango Six.
But air transport expert Strickland predicted that the measures are likely to be highly effective at limiting Russians' options.
"The amount of airspace currently closed will certainly make travel to and from Russia practically impossible from many countries," he said.