Protesting Serbs in the ethnically divided city of Mitrovica in northern Kosovo erected new barricades on Tuesday, hours after Serbia said it had put its army on the highest combat alert following weeks of escalating tensions between Belgrade and Pristina.

Serbia's defense ministry said that given the latest events in the region and Belgrade's belief that Kosovo was preparing to attack Serbs and forcefully remove the barricades, President Aleksandar Vucic had ordered Serbia's army and police to be put on the highest alert.

Kosovo called on NATO peacekeepers to remove the barricades, but said it had the capacity and readiness to act.

Kosovo and Serbia intend to join the European Union and have agreed, as part of that membership process, to resolve their outstanding issues and build good neighbourly relations.

Why are there tensions?

Kosovo unilaterally declared independence from Serbia in 2008, almost a decade after a guerrilla uprising against Belgrade's rule.

Serbia, however, still considers Kosovo to be an integral part of its territory and accuses Pristina of trampling on the rights of minority Serbs.

Ethnic Serbs, who do not recognise the Pristina government or Kosovan state institutions, account for 5 percent of Kosovo's 1.8 million people, with ethnic Albanians making up about 90 percent. The Serbs have vented their hostility by refusing to pay Kosovo's power operator for the electricity they use for example and frequently attacking police who try to make arrests.

Why fresh tensions?

Fresh ethnic tensions have errupted since December 10 when Serbs erected multiple roadblocks and exchanged fire with police after the arrest of a former Serb policeman for allegedly assaulting serving police officers during a previous protest.

The stand-off comes after months of trouble over the issue of car license plates. Kosovo has for years wanted the approximately 50,000 Serbs in the north to switch their Serbian car license plates to ones issued by Pristina, as part of the government's desire to assert authority over its territory.

On July 31, Pristina announced a two-month window for the plates to be switched over, triggering protests, but it later agreed to push the implementation date back to next year.

Ethnic Serb mayors in northern municipalities, along with local judges and some 600 police officers, resigned in November in protest at the looming switch.

Serbs in Kosovo want to create an association of majority-Serb municipalities that would operate with greater autonomy. Serbia and Kosovo have made little progress on this and other issues since committing in 2013 to the EU-sponsored dialogue.

NATO & EU roles

NATO has about 3,700 troops stationed in Kosovo to maintain the peace. The alliance said it would intervene in line with its mandate if stability in the area were jeopardized. 

The European Union Rule of Law Mission in Kosovo (EULEX), which arrived in 2008, still has around 200 special police officers there.