Tensions were high in northern Kosovo on Sunday after unknown attackers exchanged gunfire with the police and threw a stun grenade at EU law enforcers during the night.
Hundreds of Serbs, outraged over the arrest of a former police officer, gathered again early in the morning at the roadblocks erected Saturday and which have paralysed traffic through two border crossings from Kosovo to Serbia.
Although Kosovo declared independence from Serbia in 2008, Belgrade does not recognise it and encourages the Serb majority in northern Kosovo to defy Pristina's authority.
Hours after the barricades went up, police said they suffered three successive firearm attacks on Saturday night on one of the roads leading to the border.
"The police units, in self-defence, were forced to respond with firearms to the criminal persons and groups, who were repelled and left in an unknown direction," police said in a statement.
European Union police deployed in the region as part of the rule of law mission (EULEX) said they were also targeted with a stun grenade, but no officers were injured.
"This attack, as well as the attacks on Kosovo Police officers, are unacceptable," EULEX said in a press release.
EU chief diplomat Josep Borrell condemned the attacks and called on Kosovo Serbs to "immediately" remove the barricades.
"Calm must be restored... all actors must avoid escalation," Borrell tweeted.
NATO, which has deployed 4,000-strong peacekeeping mission in Kosovo under a UN Security Council mandate, blasted the "unacceptable" attacks.
"Our @NATO_KFOR mission remains extremely vigilant & fully capable of carrying out its @UN mandate in #Kosovo," NATO spokesperson Oana Lungescu tweeted.
"We call on all parties to avoid provocative actions and rhetoric & to contribute to calm & stability."
Tensions mounted after Kosovo scheduled local elections in Serb-majority municipalities for December 18, with the main Serb political party saying it would stage a boycott.
Explosions and shootings were heard earlier this week as election authorities tried to prepare the ground for the vote, while an ethnic Albanian policeman was wounded.
Shortly after the roadblocks appeared, Kosovo President Vjosa Osmani decided to postpone the elections for April 23.
The embassies of France, Germany, Italy, the UK and the US -- along with the local EU office -- welcomed the postponement, branding it a "constructive decision" which "advances efforts to promote a more secure situation in the north".
Pristina and Belgrade traded accusations over the latest incidents.
Serbian President Aleksandar Vucic said he will ask NATO peacekeepers to allow the deployment of Serbian military and police in Kosovo, although he said he believes there is "no chance of the request being approved".
Kosovo Prime Minister Albin Kurti blamed Serbia for "threatening Kosovo with aggression".
"We do not want conflict, we want peace and progress, but we will respond to aggression with all the power we have," Kurti warned on Facebook.
Serbs make up around 120,000 of Kosovo's roughly 1.8 million population, which is overwhelmingly ethnic Albanian.