Ugandan President Yoweri Museveni left crisis talks between Burundi's rival political factions Wednesday without a deal, but last-ditch efforts following months of violence continued ahead of polls next week.
Burundian President Pierre Nkurunziza's bid to stand for a third consecutive five-year term in an election next Tuesday, despite a constitutional two-term limit, has sparked months of turmoil and an attempted coup in mid-May.
"The ruling party in Burundi and the opposition parties and the civil society have agreed to negotiate expeditiously, intensively... in order to reach an agreement," Museveni told reporters as he left.
Previous United Nations-mediated efforts have all collapsed.
Ugandan Defence Minister Crispus Kiyonga took over mediation efforts started by Museveni a day earlier, in the latest bid to push stalled talks between Nkurunziza's ruling CNDD-FDD party and opposition groups.
Museveni, who was appointed mediator last week by the five-nation East African Community (EAC), has urged Burundians "to forget their past sectarian political differences".
Around 100 people have been killed in more than two months of protests, with over 158,000 refugees fleeing to neighbouring countries, according to the UN.
Gunfire and grenade explosions have hit the capital in recent nights, as has been common in recent weeks.
With the presidential election now taking place on Tuesday, there are only a few days to seal a deal between the opposing sides, but Burundi presidential communication advisor Willy Nyamitwe said there had been "positive" talks.
Key opposition leader Agathon Rwasa said talks would continue, adding that "there will be no taboo subjects, everything must be put on the table".
Polls, originally due on Wednesday, were pushed back by six days amid intense international pressure.
But a group of 17 aid agencies and rights groups warned Wednesday that was "an insufficient gesture that ignores the risk that elections could spark major violence".
Opposition groups say another term would violate a peace deal that paved the way to end a dozen years of civil war in 2006. There are fears the current crisis could plunge the impoverished, landlocked country back into civil war.
"Scores have already been killed, but this could be just the beginning of something much worse," said Ndung'u Wainaina, from the Kenya-based International Center for Policy and Conflict, a member of the coalition demanding further delays.
"In the current context of tensions and credible threats of further violence, holding elections next week could push Burundi into a much deeper crisis," Wainaina added.
Nkurunziza's ruling party scored a widely-expected landslide win in parliamentary polls held on May 29, but these were boycotted by the opposition and condemned internationally as not free and fair.
Violence has continued in Burundi, where troops clashed with suspected rebel fighters over the weekend in northern regions bordering Rwanda.
Burundian rebel general Leonard Ngendakumana -- who took part in the failed coup in May to topple Nkurunziza -- has confirmed that soldiers loyal to the coup plot were involved in the fighting.
Opposition leader Alexis Sinduhije on Wednesday said a group of exiled Burundian dissidents had created an alliance to force Nkurunziza from power, likely to be based in the Ethiopian capital Addis Ababa.
"The global aim is to get Nkurunziza out," he told France 24.
"Unfortunately, I have the impression the only way will be through violence."
Nkurunziza "can leave now and be thanked by Burundians for saving lives, or he can refuse to go and be forced out", Sinduhije added.