NATO troops stepped up patrols in Kosovo on Monday near border crossings which have been blocked by local Serbs angered by a ban on cars with Serbian licence plates entering the country in a mounting confrontation.
Video footage showed NATO armoured vehicles moving close to the barricades made of trucks and other vehicles on the Kosovo-Serbia frontier, as Serbian government jets flew overheard.
Serbs from Kosovo's north have blocked two main roads since the number-plate ban went into force on Monday last week, exacerbating tensions between the two countries and prompting calls for calm from NATO and the European Union.
Under the ban, all drivers from Serbia must now use temporary printed registration details that are valid for 60 days.
The Kosovo government says it is in retaliation for measures in force in Serbia against drivers from Kosovo since 2008, when Kosovo declared independence from Serbia. Serbia does not recognise Kosovo's independence and therefore its right to take official actions such as registering cars.
The confrontation has already boiled over into violence - two interior ministry buildings in northern Kosovo, including a car registration office, were attacked on Saturday, police said.
Serbia has also begun military manoeuvres near the border and started flying military jets above the crossing in protest.
Kosovo's NATO mission - whose 5,000 troops oversee a fragile peace in Kosovo - said on Monday it had "increased the number and duration of routine patrols Kosovo-wide, including in northern Kosovo".
Serbia lost control over Kosovo in 1999 after NATO bombing.
Kosovo declared independence in 2008, backed by the United States, Britain and most EU member states, but its membership of the United Nations is blocked by Russia, Serbia's traditional ally.
Belgrade and Pristina committed to an EU-sponsored dialogue in 2013 to resolve outstanding issues, but little progress has been made.