BELGRADE — Unmarked drones appeared on the Kosovo-Serbia border “observing the barracks and the positions” of the Serbian army, Belgrade authorities claimed, prompting Serbian President Aleksandar Vučić to order the country’s fighter jets to intercept them.

The drones left the Serbian airspace before the warplanes reached their targets, public broadcaster RTS reported.

Prime Minister Ana Brnabić said in an interview with RTS on Tuesday evening that the drones were spotted while observing one military position on the Serbian side of the border and two barracks, adding that she did not want to speculate on the origin of the unmanned devices.

She emphasized that the order was to shoot down the drones on sight, and the Serbian authorities would be further monitoring the area.

“If these things keep happening, we will take down any such aircraft, just like any other country would,” Brnabić added. The drone sightings could not be independently verified.

The news of a drone incursion came after Vučić placed Serbian troops on the border with Kosovo on a heightened state of alert over a row about car license plates that has threatened to further escalate tensions between the two Balkan countries.

Earlier, authorities in Pristina issued a warning demanding that members of the ethnic Serbian minority living in Kosovo, a former province of Serbia, replace their vehicle registration plates with Kosovar ones.

The warning came despite calls from the European Union and the United States to postpone any such requirement.

In response to Kosovo’s move, Serbian Defense Minister Milos Vučević said that Vučić, who is the supreme commander of Serbia’s armed forces, placed the military in a state of “readiness”.

“We are not getting ready for a war, but we must not be unprepared,” Vučević said. “We are ready to continue our dialogue with Pristina.”

It was not clear what the state of readiness meant in practice. The Serbian leader has been prone to saber rattling when tensions get high with Kosovo, whose independence Belgrade does not recognize.

The EU has told Kosovo and Serbia that the two must normalize ties if they want to advance toward membership in the 27-nation bloc.

Brussels and Washington recently have stepped up mediation efforts, fearing uncertainties over the war in Ukraine and Serbia’s close ties with Russia could aggravate matters.

Trouble started brewing in the summer over Serbia’s and Kosovo’s refusal to recognize each other’s identity documents as well as car license plates.

Kosovo Serbs living in the north erected roadblocks, sounded air raid sirens and fired guns into the air in protest of the move.

In June, Kosovo passed a law requiring vehicles with Serbian number plates to replace them with Kosovo ones.

But following pressure from the EU and the US and the tension in the largely Serb-populated north, it agreed to postpone implementation until Nov. 1.

As the measure came into effect Tuesday, Kosovo authorities said enforcement would be gradual.

For the first three weeks in November, drivers who fail to comply will receive warnings. After that, they will be fined, and from late January, only vehicles with new temporary plates will be allowed to circulate.

The final date after which no old license plates will be allowed, and drivers risk the impounding of their vehicles, has been set for April 21.

Kosovo’s Interior Minister Xhelal Sveçla urged ethnic Serbs Tuesday to comply and not remain “pawns to political interests”.

Last week, Sveçla said only 20 Serbs had changed plates. At least three attacks on those who decided to make the switch have been reported, with cars said to have been torched.

Kosovo’s 2008 independence has been recognized by most EU countries and the US, while Serbia has relied on support from Russia and China for its bid to retain the former province.

Belgrade lost control over Kosovo in 1999 after NATO bombed the country to stop its brutal crackdown on ethnic Albanian rebels, following decades of repression by the regime of strongman President Slobodan Milošević.

NATO has a peacekeeping mission in Kosovo, and any Serbian military intervention would risk reviving widespread tensions that plunged the Balkans into a series of wars in the 1990s.

Belgrade repeatedly said it planned to respect the 1999 Kumanovo Agreement — which effectively ended the conflict in Kosovo — with the Serbian side withdrawing from its former province and pledging not to enter its territory with its military in the future. — Euronews