The Serbian army says it is at its “highest level of combat readiness” after weeks of escalating tensions between Serbia and Kosovo.
President Aleksandar Vucic says he will “take all measures to protect our people and preserve Serbia”.
The sabre-rattling is louder than ever and follows some media claims in Serbia that Pristina is preparing “an attack” on ethnic Serb areas in north Kosovo.
The government in Pristina has made no comment about the allegations.
But it has previously accused Mr Vucic of playing “games” to stir up trouble.
Kosovo, which has an overwhelmingly ethnic Albanian majority, broke away from Serbia after a war in 1998-99. Serbia does not recognise Kosovo as an independent state, nor do the ethnic Serbs who live there.
Belgrade accuses Kosovo of plotting “terrorism against Serbs” in areas where some 50,000 ethnic Serbs live.
In return, Pristina says Belgrade is behind “paramilitary formations” that set up roadblocks in majority Serb areas of north Kosovo on 10 December.
The European Union has been attempting to mediate. The 27-member bloc is calling for “maximum restraint and immediate action” and for the leaders of Serbia and Kosovo to “personally contribute to a political solution”.
Belgrade has increased its armed presence on the border in recent days after conflicting reports about a shooting incident in which no-one was hurt. Reports from Belgrade alleged ethnic Serbs came under attack, but the claim was rejected by Kosovo authorities in Pristina.
Nevertheless, Serbia appeared to use the reports to justify intensifying its military presence on the border.
So far, hostilities have been limited to harsh words, then on Monday, Serbia placed its army on combat alert. Kosovo has warned that if Nato’s KFOR peacekeeping force does not remove the roadblocks, then it will take matters into its own hands.
Nato, which has some 3,700 peacekeeping troops in Kosovo, has called on all parties to avoid provocations. For years, its KFOR troops have played a crucial role in guaranteeing the peace. The European Union, which has a rule-of-law mission in Kosovo, has warned it will not tolerate attacks on EU police or criminal acts.
It is a worryingly combustible situation. Tensions between Belgrade and Pristina have ebbed and flowed since Kosovo’s unilateral declaration of independence from Serbia in 2008.
But they have been running particularly high over the past few months, with ethnic Serbs withdrawing all co-operation with Kosovo authorities.
