The dialogue set with the Burundian government has been suspended by opposition activists who condemned the assassination of Zedi Feruzi, the opposition leader of the Union for Peace and Development (UPD) Zigamibanga party, and his bodyguard in the capital Bujumbura.
Condemning the "awful act," activists said in a statement they were "suspending participation in dialogue with the government" after Zedi Feruzi, the leader of the UPD was shot dead on Saturday.
The activists accused the Burundi President Nkurunziza of “planning to physically eliminate” leaders who are against his campaign. A journalist who witnessed the attack said the gunmen were clad in uniforms similar to those worn by the presidential guard.
The presidency has denied any involvement, saying it was "shocked" by the attack, in which a police bodyguard was also killed. It said the incident should be urgently investigated "so the guilty are brought to justice."
UN Secretary General Ban Ki-moon also condemned the latest violence and appealed for "calm and restraint."
"These acts of violence constitute a stark reminder of the need for all Burundian political leaders to address the current political crisis with the highest sense of responsibility and to place peace and national reconciliation above partisan interests," Ban's office said.
Burundi's President Pierre Nkurunziza delayed parliamentary elections which were set for May 26 by a week, after nearly a month of anti-government demonstrations and a failed military coup in the east African state.
The constitution limits the presidency to two terms in office, but Nkurunziza's supporters say he can run again because his first term was picked by lawmakers and not by elections.
Despite the demonstrations against his third term bid, Burundi's Constitutional Court approved President Pierre Nkurunziza’s candidacy in a controversial decision the exiled head of the body claims was made through coercion.