BUJUMBURA - UN Secretary-General Ban Ki-moon urged Burundi’s government and its opponents not to let violence derail United Nations-sponsored talks, after the killing of an opposition leader prompted some opponents to say they would walk out.

Ban condemned the killing by unidentified gunmen late on Saturday of Zedi Feruzi, the head of the UPD party, who opposed President Pierre Nkurunziza’s bid for a third term. Nkurunziza’s decision to run has sparked the worst crisis in the small African country since the end of an ethnically charged civil war in 2005. The U.N. special envoy to the region, Said Djinnit, and other international mediators have been hosting initial dialogue between rival sides to end the crisis. Opposition parties and civil society activists say Nkurunziza’s re-election bid violates a two-term limit in the constitution and a peace deal that ended civil conflict.

Protesters have regularly clashed with police in the past month and unrest provoked a failed military coup on May 13. The president, who has called protests an “insurrection”, points to a constitutional court ruling that said his first term, when he was picked by parliament not a popular vote, did not count. He has shown no signs of backing down from his bid.

The U.N. secretary-general “calls on the parties to the consultative political dialogue not to be deterred by those who, through violence, seek to prevent the creation of an environment conducive to peaceful, credible and inclusive elections in Burundi,” his office said in a statement. In the wake of Feruzi’s killing, Anshere Nikoyagize, the head of the civil society group Ligue ITEKA, told Reuters that civil society groups and opposition parties would not attend the dialogue, which began this month. He did not give more details.

Other partners helping broker the dialogue include the African Union and regional African representatives. Burundi’s crisis has set the region that has a history of ethnic conflict on edge. More than 110,000 Burundians - about 1 percent of the country’s population - have already fled across the border for fear violence will spread outside the capital.

Willy Nyamitwe, presidential media adviser, told Reuters that another round of dialogue was planned for Sunday. But the fate of those talks was not clear given the threats of a boycott. “The main objective is to find ways of coming out of this situation,” he said. “This situation is going out of control.” There has been little sign of progress in bridging differences. The president has insisted he will follow his party’s call to seek another five years in office, while opponents say protests will continue until he ends his bid.