MITROVICA, Kosovo (AP) — Serbia's president on Saturday pledged to help build a lasting peace in Kosovo days after the killing this week of a moderate Kosovo Serb politician fueled fears of instability in the Balkans. Amid tight security, Aleksandar Vucic arrived for a two-day visit that officials said was designed to ease concerns among Kosovo's Serbs following the slaying Tuesday of one of their leading politicians, Oliver Ivanovic. Waving Serbian flags, hundreds of people cheered Vucic as he arrived in the Serb-dominated, northern part of the former Serbian province that declared independence in 2008.