The African Union (AU) has said that there may be "catastrophic consequences" for Burundi and the region if political differences are not resolved peacefully.
This comes a day after Burundi’s former army chief of staff Jean Bikomagu was assassinated, further indicating the possibility of renewed conflict in the country which has witnessed violence since April over President Pierre Nkurunziza's decision to seek a third term.
This despicable act, and multiple other acts of violence recorded in recent months, illustrates yet again the gravity of the situation in Burundi
Nkurunziza won the July presidential elections despite deadly violence and boycott by the opposition parties and civil society groups.
AU Commission chief Nkosazana Dlamini-Zuma on Sunday called for "utmost restraint" from all sides.
"This despicable act, and multiple other acts of violence recorded in recent months, illustrates yet again the gravity of the situation in Burundi - and the real risk of seeing a further deterioration with catastrophic consequences both for the country itself, and for the whole region," said Dlamini-Zuma.
President Nkurunziza, a 51-year-old former sports teacher and born-again Christian, was a Hutu rebel leader during the civil war. The war pitted rebels from the majority Hutu people against an army dominated by the minority Tutsi.
Colonel Jean Bikomagu, a key figure in the former Tutsi-dominated army during the conflict which raged between 1993 and 2006, was gunned down on Saturday in his car by unidentified assailants outside his home in the capital Bujumbura as he returned from church.
The 13-year civil war left at least 300,000 people dead.
The murder comes less than two weeks after the assassination by rocket attack, of feared top general Adolphe Nshimirimana, widely considered to be the nation’s de-facto security chief.
Presidency spokesman Willy Nyamitwe commented on the events, saying: "Another black weekend in Burundi. Blackened by the assassination of a senior member of the Burundian army in retirement, Colonel Jean Bikomagu ...Very sad, very shocking."
The escalation of violence in the region has increased fears that the small country in Africa's Great Lakes region could descend back into a state of conflict, with many afraid of pro-government retaliations after Nshimirimana's death.
Dlamini-Zuma called for the "need for dialogue and consensus to find a peaceful and lasting solution to the current crisis."
The AU commission chief also said that the pan-African body continued to support mediation efforts led by Ugandan President Yoweri Museveni.