NATO troops secured a hostile strip of north Kosovo today after Serb riots in Mitrovica killed one Ukrainian U.N. police officer and forced the pullout of U.N. personnel from the Serb stronghold. The number of people injured in the clashes has risen to 130, half of whom were civilians. The violence, sparked by a U.N. police operation to retake a U.N. court seized by protesting Serbs, was the worst since Kosovo’s ethnic Albanian majority declared independence from Serbia on February 17. Professor Stefan Wolff of Nottingham University in the UK called the latest violence worrying and said it casts doubt on plans to deploy a European Union police mission.