BELGRADE, Serbia (AFP) — Serbian armed forces were on "the highest level" of alert, Defense Minister Milos Vucevic has said, highlighting the Balkan country's increasingly strained relations with neighboring Kosovo over recent shootings and blockades.

Kosovo declared independence from Serbia in 2008, but Belgrade has refused to recognize it and encouraged Kosovo's 120,000 ethnic Serbs to defy Pristina's authority — especially in the north where ethnic Serbs make up the majority.

The Serbian army has been put on a heightened state of alert over tensions with Kosovo multiple times in recent years — the last time in November after claiming that several drones entered Serbian airspace from Kosovo.

On 10 December, Serbs in northern Kosovo set up barricades to protest against the arrest of an ex-policeman suspected of being involved in attacks against ethnic Albanian police officers.

The blockades have coincided with a rise in reported shootings, the latest on Sunday, according to NATO-led peacekeeping force KFOR.

"Serbia's president… ordered the Serbian army to be on the highest level of combat readiness, that is to the level of the use of armed force," Defence Minister Milos Vucevic said in a statement.

He added that President Aleksandar Vucic also ordered the special armed forces to be beefed up from the existing 1,500 to 5,000.

Serbia's interior ministry also announced that "all units (will) immediately come under the command of the chief of general staff."

These orders from Vucic come after army chief General Milan Mojsilovic was dispatched to the border with Kosovo on Sunday.

"The situation there is complicated and complex," Mojsilovic said Sunday.