Serbian armed forces were on "the highest level" of alert, Defence Minister Milos Vucevic said late on Monday, amid the Balkan country's increasingly strained relations with neighbouring Kosovo over recent shootings and blockades.
Kosovo declared independence from Serbia in 2008, but Belgrade refuses to recognise it and encourages 120,000 ethnic Serbs to defy Pristina's authority, especially in the north where they make up the majority.
The Serbian army has been put on heightened alert over tension with Kosovo several times in recent years, with the last time being November after claiming that drones entered Serbian airspace from its breakaway province.
On December 10, Serbs in northern Kosovo set up barricades to protest against the arrest of a former policeman suspected of being involved in attacks against ethnic Albanian officers.
The blockades have coincided with a rise in reported shootings, the latest coming on Sunday evening, 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," Mr Vucevic said.
He said President Aleksandar Vucic also ordered the special armed forces to be increased from the existing 1,500 troops to 5,000, Vucevic said.
Serbia's Interior Ministry also announced "all units [will] immediately come under the command of the chief of general staff".
The orders from Mr Vucic come after army chief Gen Milan Mojsilovic was sent to the border with Kosovo on Sunday.
Northern Kosovo has been especially on edge since November, when hundreds of ethnic Serb workers in the Kosovo police and the judiciary, including judges and prosecutors, walked off the job in protest against a decision to ban Serbs living in Kosovo from using Belgrade-issued licence plates.
The policy was scrapped by Pristina but the mass walkouts created a security vacuum.
Pristina tried to schedule December 18 local elections in Serb-majority municipalities, but they were postponed after the announcement caused widespread outrage and the main Serb political party said it would stage a boycott.
Then on December 10, the former policeman suspected of involvement in attacks against ethnic Albanian officers was arrested, outraging ethnic Serbs who erected barricades that paralysed traffic around two border crossings.
Just hours after the barricades were erected, Kosovar police said they were victims of three gun attacks on one of the roads leading to the border.
Kfor, which has increased its presence and patrols in the region in recent months, said the latest bout of violence came on Sunday, when shots were fired in the direction of Latvian soldiers embedded with the force, Kosovo's Defence Ministry tweeted.
Kfor said it was investigating the incident, in which there were "no injuries or material damage".
Serbian Prime Minister Ana Brnabic said last week the situation with Kosovo was "on the brink of armed conflict".
Kosovo's security council, which met on Monday, blamed Serbia for the latest deterioration in relations, saying its northern neighbour "is acting with all available means against the constitutional order of the Republic of Kosovo".
Serbs comprise about 120,000 of Kosovo's 1.8 million population.