PRISTINA, Kosovo — Six Serbs accused of involvement in the murder of a Kosovo Serb political party leader pleaded not guilty at the start of a much-anticipated trial in a Kosovo court on Friday.

Oliver Ivanovic, once a hardline nationalist who turned a moderate politician, was gunned down in front of his party office in the Serb-controlled northern part of Mitrovica on January 16, 2018.

The killing increased tensions between Kosovo and Serbia. Kosovo declared independence from Serbia in 2008 but Serbia does not recognize its former province's statehood.

The six defendants are accused helping an organized criminal group to carry out the murder. Three alleged gang leaders have fled to Serbia and Kosovo police have issued an arrest warrant for them over the killing.

At the time of his death, Ivanovic was on trial for allegedly ordering the murder of Kosovo Albanians during the war in Kosovo in 1999. He had pleaded not guilty.