The head of the UN administration in Kosovo has condemned a drive-by shooting on Saturday in which two Serbs were killed and two wounded.
Soren Jessen-Petersen said no efforts would be spared to catch the perpetrators. The incident happened near the southern town of Strpce.
The attacks follow a period of calm in UN-administered Kosovo. Last year 19 people died in anti-Serb riots there.
The Albanian majority wants Kosovo to break away from Serbia and Montenegro.
Mr Jessen-Petersen said he was "shocked and appalled" by Saturday's shooting and urged police "to find the perpetrators of this loathsome crime".
In March 2004, 19 people were killed and hundreds more injured in an explosion of anti-Serb violence in Kosovo. It was the worst violence in Kosovo since 1999, when a Nato bombing campaign ended a Serb crackdown.
Though governed by the UN, the province is formally still under Belgrade's rule and its final status has yet to be resolved.
A 17,000-strong Nato-led peacekeeping force is deployed in Kosovo.