Mitrovica, a city of 71,000 in the predominantly Serbian northern part of Kosovo, was the scene of a bombing Sunday in which an Albanian man was killed his two children injured when an explosive device went off on his apartment window, the Serbian Beta News Agency reported.
Kosovo Interior Minister Bajram Rexhepi said the incident had the look of a professional job.
Other ethnically charged incidents happened shortly thereafter.
Reports indicated a 66-year-old Kosovo Serb sustained severe head and chest injuries when he was assaulted by three ethnic Albanians in Mitrovica's Suvi Do neighborhood.
The situation worsened Monday when the Albanian-dominated Kosovo Police Service attempted to move three police checkpoints north across the Ibar River and set them up in the city's predominantly Serbian area.
They were met with violence when residents overturned one of boxes and broke its windows. The other checkpoints were taken back across the river by the KPS, the news agency said.
The violence brought a condemnation Monday from the Organization for Security and Cooperation in Europe, which monitors human rights and the rule of law under in the former Serbian province under a U.N. mandate.
Edward Joseph, the OSCE's acting head of mission in Kosovo, decried the police checkpoint assault as well as Sunday's bombing.
"My colleagues and I are shocked by the despicable attack in which Mr. Selver Hajredinaj was killed yesterday in Mitrovica," Joseph said. "Any incident that affects the communities is a great cause for concern. I wish Mr. Vladimir Denovic a speedy recovery from the injuries he sustained in a separate incident yesterday in Mitrovica."
EU Special Representative in Kosovo Samuel Zbogar also urged calm.
"I was shocked when I heard about the bombing which killed Selver Haradinaj in Mitrovica … I hope that this investigation will be carried out quickly and successfully," Zbogar said in a statement. "Meanwhile, I appeal to all communities to remain calm and leave the case in the hands of police."
"Further acts of violence cannot, and must not, be tolerated. Such acts can only threaten Kosovo's peace and stability," he added.
Another call for calm came from U.N. envoy to Kosovo Farid Zarif, who said further acts of violence couldn't be tolerated as they "can only threaten Kosovo's peace and stability," Radio Free Europe/Radio Liberty reported.
Residents say they fear Mitrovica, about 22 miles northwest of Pristina, is a tinderbox for ethnic tensions and that the violence could quickly escalate.
"I am afraid that we (Albanians) in the south might be attacked as well," 70-year-old Rrahman Krasniqi told the U.S.-sponsored Southeast European Times online newspaper. "I hope those acts will be stopped and no one gets hurt anymore. We are tired of violence and conflict."
Kosovo Police Service spokesman Besim Hoti said an investigation into the bombing was ongoing and that his aim was to provide greater security for Mitrovica and reduce tension there.
"We have an extremely tense situation [and heightened] concerns of residents -- not just in the north, Albanian and Serbian residents -- but all other residents in the municipality and beyond," Hoti told RFE/RL.
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