"We urge all sides to avoid unsafe rhetoric and remain calm at this sensitive time, and recommit themselves to continue the work toward the normalization of relations and improvement of the lives of the citizens of Kosovo and Serbia", the United States ambassador said.
Oliver Ivanovic was shot in the chest at point blank range outside his party's headquarters in the city of Mitrovica in Kosovo. The shooter is still unknown, though police said they believe a burned out vehicle found in the city could be linked to the killing.
Known as a relative moderate among mainly ultra-nationalist Kosovo Serb politicians, Ivanovic became one of the chief interlocutors for NATO, UN and European Union officials based in Kosovo to help steer it towards stable democracy and rule of law. He was convicted, but his conviction was overturned on appeal and he was released pending the outcome of the new trial two years later.
"Serbia will take all necessary measures, and I promised the same to Oliver Ivanovic's wife, and we will find the killer or killers", Vucic said in a statement following an emergency meeting of the National Security Council.
Vucic said he asked European Union and United Nations officials in Kosovo to include Serbian security forces to investigate Ivanovic's killing.
Belgrade does not recognise the independence of Kosovo, its former province predominantly populated by ethnic Albanians.
Marko Djuric, the Serbian government's official dealing with Kosovo, described the incident as a "criminal act of terror".
He was considered a moderate politician, following a new political trend, and took part in all common avtivities with Albanian politicians in which he spoke in Albanian. While commenting on the incident in a DW interview, he said the auto burning had been a "message" aimed to scare him into curbing his political activity.
Ivanovic, the 64-year-old head of the Kosovo Serb Freedom, Democracy, Justice party was due to attend EU-mediated talks between delegates from Kosovo and Serbia in Brussels.
The Kosovo War of 1998-1999 between the ethnically Kosovar Albanian forces and the former Yugoslavia, then consisting of Serbia and Montenegro, ended after the UN-backed global intervention following NATO's bombing of Yugoslavia's forces.
About 120,000 of Kosovo's 1.8 million inhabitants are ethnic Serbs.
Serbian officials dismissed the accusations against Ivanovic and said the process against him was staged.
However, after the assassination, the Belgrade delegation walked out of the talks, according to local media in Belgrade.