BELGRADE, Serbia (AP) — Serbian authorities have banned Montenegro’s national carrier from operating flights out of Belgrade after the small nation's government excluded Serbia from a list of countries with which Montenegro will reopen its borders after declaring an end to its coronavirus outbreak.

The Serbian Civil Aviation Directorate said Montenegro Airlines planes cannot land at Belgrade Nikola Tesla Airport as of Wednesday. The aviation agency said unrestricted travel between the two countries has been “seriously violated” by Montenegro’s decision to ban Serbs from entering Montenegro.

The flight and border entry bans come amid deepening tensions between the former Balkan allies, which existed as one state before Montenegro split off through a 2006 referendum.

Montenegro Airlines planned to resume flights to the Serbian capital on June 1. Air Serbia, still has plans to fly to two destinations in Montenegro starting June 7.

Montenegro declared itself “coronavirus-free” as the first country in Europe with no more active COVID-19 cases. Before then, the country recorded a total of 324 confirmed virus cases and nine deaths. Serbia has recorded over 11,000 confirmed cases, including nearly 240 deaths.

The Montenegrin government announced Monday that as of June 1 it would permit entry to the nationals from nine European countries. It said the selection criteria was having no more than 25 active coronavirus-infected patients per 100.000 inhabitants.

The list includes Croatia, Slovenia, Austria, Germany, Poland, the Czech Republic, Hungary, Albania and Greece, but does not include neighboring Serbia or Bosnia.

Without saying that Serbia meets Montenegro’s entry criteria, Belgrade denounced the decision as political and hostile. Montenegrin officials tried to dampen the Serb anger by saying the list of nationals who will be allowed to enter has not yet been finalized.

Populist Serbian President Aleksandar Vucic said on Wednesday the flight ban is the “most legal and polite” response to the Montenegrin travel action, adding that Serbia won’t close its borders to Montenegro’s citizens as it would hurt Serbs who live there.

Montenegrin Prime Minister Dusko Markovic responded by saying his government's decision was not political, but based on health risks.

“We were not angry when Serbia closed the border with us, at the beginning of this outbreak. It was the first border they closed," Markovic said.

Tensions between Serbia and Montenegro have escalated since late last year, when the Montenegrin parliament adopted a religious law which the Serbian Orthodox Church said would strip it of its property in Montenegro. Montenegrin officials have denied the allegation.

The Serbian government has openly supported a series of protests by Serbs in Montenegro who want the law revoked.

Montenegrin officials have accused Belgrade and Moscow of trying to destabilize the nation's pro-Western government by launching a major propaganda campaign that fueled the protests. Montenegro joined NATO in 2017 despite strong opposition from another historic Slavic ally, Russia.

Both Belgrade and Moscow have repeatedly denied meddling in Montenegro's internal affairs.