Bujumbura - Ugandan President Yoweri Museveni left crisis talks between Burundi's rival political factions on 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 all collapsed.
Ugandan Defence Minister Crispus Kiyonga took over mediation efforts started by Museveni a day earlier, 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), had earlier 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 were heard overnight on Tuesday in the capital, as has been common in recent weeks.
With the presidential election now taking place on Tuesday, there are only a few days to succeed.
But Burundi presidential communication advisor Willy Nyamitwe said there had been "positive" talks, while key opposition leader Agathon Rwasa said talks would continue.
"There will be no taboo subjects, everything must be put on the table," Rwasa said.
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.
Parliamentary polls, in which Nkurunziza's ruling CNDD-FDD scored a widely-expected landslide win, were held on May 29, but boycotted by the opposition and internationally condemned.
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 and rights groups argue that weeks of protests and a violent crackdown by security forces mean free and fair elections are impossible.