Serbian President Aleksandar Vučić has met his national security council as tensions rise in Kosovo between the authorities there and ethnic Serbs.
On Saturday a stun grenade was thrown at EU police in north Kosovo, where Serbs form a majority, and local police exchanged fire with unknown groups.
Ethnic Serbs set up road blocks after Kosovan police were deployed in a dispute over car number plates.
Serbia refuses to recognise Kosovo as an independent state.
Kosovo, which has an overwhelmingly ethnic Albanian majority, broke away from Serbia after a war in 1998-99.
Nato, which has peacekeeping troops in Kosovo, called on all parties to avoid provocations. The EU has done likewise, warning that it will not tolerate attacks on EU police or criminal acts.
The latest unrest was triggered by the arrest of a Serb former police officer on Saturday.
Speaking to Reuters news agency after meeting his national security council, Mr Vučić said he wanted to defuse the situation.
Kosovo Prime Minister Albin Kurti described the protesters as “criminal gangs” and asked the Nato-led peacekeeping Kosovo Force (KFOR) to remove the barricades.
Kosovo police said they had come under fire in different locations close to a lake bordering Serbia late on Saturday, with officers returning fire in self-defence.
Kosovo deployed police to majority-Serb areas – which do not recognise the Kosovo government in the capital Pristina – at the end of last week. This heightened tensions further, after they were already raised for months over a row about vehicle license plates.
Mr Vučić said the use of police violated previous peace agreements. He said he planned to ask Nato peacekeepers for permission to send Serbian police and troops to the area, though acknowledged there was little chance of this being granted.
Kosovo declared independence in 2008, following the bloody year-long war a decade earlier. Serbia does not recognise it as a sovereign state, however does accept the governing authority under an agreement brokered in 2013 to normalise relations.
Of the 1.8 million people living in Kosovo, 92% are Albanians and 6% are Serbians.
The police officer arrested on Saturday was one of some 600 ethnic Serbs who resigned from the force in November in protest after authorities in Kosovo said they would require Serbs to swap Serbian licence plates dating to before the war for Kosovan ones.
Some 50,000 people in majority-Serb areas have so far refused to use Kosovo number plates because they do not recognise its independence from Serbia.
A deal brokered by the EU at the end of November saw an uneasy agreement reached between the two sides, with Kosovo dropping plans to fine people who did not swap the plates, and Serbia agreeing to stop issuing registrations with the initials of towns in Kosovo.