Serbian President Aleksandar Vucic has put his country's army on high combat alert and ordered its units to approach the border with Kosovo, the Tangug news agency reported on Friday.
Vucic's orders come after Serbs in the northern Kosovo municipality of Zvešan clashed with Kosovo police who were trying to help the newly elected Albanian mayor enter his office.
Serbs, who make up the majority of the region, boycotted the local elections.
Local media reported that Kosovo police fired tear gas canisters at crowds gathered in front of the town hall.
The Serbian president vowed in March not to recognize Kosovo at all after refusing to sign an EU-backed deal to normalize relations between the former rivals.
Serbia refuses to recognise Kosovo, a predominantly Albanian province that declared independence in 2008.
The question of Kosovo continues to occupy the attention of some 6.7 million Serbs, who consider the region the national and religious cradle of their country.
Many members of Kosovo's 120,<> Serb minority refuse to pledge allegiance to Pristina with Belgrade's encouragement, especially in northern Kosovo near the border with Serbia, where frequent clashes, demonstrations and sometimes violence.