Serbian troops were on heightened alert at the border with Kosovo, the defence ministry has said, accusing the country of "provocations" after it deployed special police to the area.
The special police units were sent to two border crossings in the north of Kosovo, an area mainly populated by minority ethnic Serbs who reject the authority of the ethnic Albanian-led government in Pristina.
The deployment, which angered the Serbs, came after the Kosovo government last Monday decided to require drivers with Serbian registration plates to put on temporary ones when entering Kosovo.
Hundreds of ethnic Serbs have staged daily protests against the decision.
They parked trucks and other vehicles along the roads to block traffic toward the border crossings.
"After the provocations by the (special police) units... Serbian President Aleksandar Vucic gave the order to heighten the alert for some Serbian army and police units," the defence ministry said in a statement.
Serbian fighter jets again overflew the border region on Sunday after several sorties on Saturday, an AFP correspondent saw.
Early on Sunday, Serbian Defence Minister Nebojsa Stefanovic visited troops at two military bases where they are on alert, including one that is just a few kilometres (miles) from the border.
Belgrade designates border crossings between Serbia and Kosovo as "administrative" because it does not recognise its former province's independence, unilaterally declared in 2008.
Belgrade's position is that the decision to mandate temporary plates implies its status as an independent nation.
Serbian ally Russia also does not recognise Kosovo's independence, but most Western countries do, including the United States.
Albania, for its part, said it was "concerned by the escalation of the situation" and called on Serbia to withdraw its troops from the border.
President Osmani cut short a visit to NY
Kosovo President Vjosa Osmani cut short a visit to New York for the UN general assembly "because of developments in the north of the country", her cabinet said.
Kosovo's declaration of independence came a decade after a war between ethnic Albanian guerrillas and Serbian forces which claimed 13,000 lives, mostly ethnic Albanians.
The United States and the European Union have called for a de-escalation of tensions and for the two sides to return to normalisation talks, which the EU has mediated for about a decade.
The Serbian president said the normalisation process can resume only if Kosovo withdraws the special police forces from the north.