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.Licence plate row continuesThe 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 licence 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.International community asks for postponementIn June, Kosovo passed a law requiring vehicles with Serbian number plates to replace them with Kosovo ones.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."