An apparent bomb planted outside an apartment window killed an ethnic Albanian man and wounded two of his children Sunday in the ethnically-tense Kosovo city of Mitrovica.

Police are investigating the incident, but Kosovo authorities are already calling it an act of terrorism

The top European Union official in Kosovo urged people to stay calm, telling the Associated Press it is best to leave the matter to the police.

Mitrovica in northern Kosovo is split between ethnic Albanian and Serbian neighborhoods. Many of the Serbs refuse to recognize Kosovo's government. Tension between the two sides occasionally boils over into violence.