International concern has been raised over a Kosovo police bid to take over two border crossings in the ethnic Serb north in which one officer was killed.
Kosovo police units, who came under fire, pulled back after Serbs refusing to recognise their authority took up arms and mounted roadblocks.
Nato-led peacekeepers moved into the area to calm the situation.
The US and EU criticised the Kosovan government for acting without consulting international bodies.
Serbia refused to recognise the formal secession declared by the ethnic Albanian majority in 2008, and maintains close ties with the region north of Mitrovica, where ethnic Serbs have their main enclave.
Kosovo's government in Pristina says its police force acted on Monday night after its decision last week to ban Serbian goods from entering Kosovan territory, in response to an equivalent ban by Serbia.
Since the war in 1999, Kosovo has been controlled by international bodies, including a 3,000-strong EU rule of law mission, which oversee the territory's own new authorities.
'We do not approve it'
Nato's Kosovo commander, Maj Gen Erhard Buehler, is believed to have secured a withdrawal of the police units from the border crossings as part of a deal reached with the Kosovan authorities, but Pristina denied any deal had been reached, the Associated Press reports.
Ethnic Albanian policeman Enver Zymberi died late on Tuesday from injuries he suffered when his unit was ambushed, Kosovo police said. Another officer was reportedly injured.
Kosovan Prime Minister Hashim Thaci defended the police operation at a news conference in Pristina, saying: "We cannot stay indifferent and tolerate forever that a part of our territory is to be a black hole not only of Kosovo but of the Europe.
"We cannot tolerate forever that our sovereignty is violated."
Policing in north Kosovo has until now been largely conducted by EU officers and Serb members of the Kosovo police.
Oliver Ivanovic, Serbia's state secretary for Kosovo, warned the situation was "extremely tense" and described Pristina's police operation as a "hastily made, unexplainable gesture".
A US state department spokeswoman said: "The United States regrets that last night's action by the Kosovo government... was not co-ordinated with the international community."
The US, she added, urged both Pristina and Belgrade "to continue to work urgently toward a de-escalation of the current situation".
In Brussels, a spokeswoman for EU foreign policy chief Catherine Ashton said: "We believe the operation by the Kosovo authorities was not helpful. We do not approve it."