Because of the stationing of special units of the Kosovar police in the border region, perceived as a “provocation”, Serbia has put its army on heightened alert.

After the "provocation" by the special forces in Kosovo, Serbia's President Aleksandar Vucic gave the order to put some units of the Serbian army and police on heightened alert, the Ministry of Defense announced in Belgrade on Sunday.

The EU and NATO called on both sides to exercise restraint.

Serbian Defense Minister Nebojsa Stefanovic visited troops in two army bases on Sunday.

One of them is near the border with Kosovo.

Serbs in Kosovo block border crossings

The special forces of the police of Kosovo had previously been posted at two border crossings in the north of the country.

In the area there are predominantly members of the minority ethnic Serbs who do not recognize the authority of the ethnic Albanian-led government in the capital Pristina.

The police units were stationed in accordance with an order from the Kosovar government, according to which motorists with Serbian license plates are obliged to put up provisional license plates when entering Kosovo.

Hundreds of ethnic Serbs have protested against the requirement every day since the order last Monday and used vehicles to obstruct traffic at the border crossings.

From Belgrade's point of view, license plates from Kosovo imply its status as an independent nation.

Kosovo broke away from Serbia in 2008.

However, the government in Belgrade still does not recognize independence and regards Kosovo as a breakaway province.

Serbia's ally Russia, unlike most Western countries, including the USA, does not recognize Kosovo's independence.

Kosovo's President Vjosa Osmani broke off her visit to the UN in New York in view of the developments in her country.

Albania expressed concern about the recent "escalation of the situation" and called on Serbia to withdraw its troops from the border region.

The EU foreign affairs representative Josep Borrell called on both sides to calm down the situation.

The special police forces would have to be withdrawn immediately.

"Any further provocation or unilateral and uncoordinated action is unacceptable," he said.

NATO Secretary General Jens Stoltenberg also called for moderation.

"It is important that Belgrade and Pristina exercise restraint and resume the dialogue," he said on Twitter after a phone call with the Serbian President and the Kosovar Prime Minister Albin Kurti.