Burundi's government has defied opposition demands for President Pierre Nkurunziza to end a third-term bid for power, as the UN warned the country risked being "catapulted" back into civil war.
"This decision is non negotiable," Philippe Nzobonariba, a government spokesman, said on Tuesday.
Nkurunziza's opponents say his candidacy is unconstitutional and goes against the 2006 Arusha peace deal that ended 13 years of civil war.
About 40 people have died and scores more have been injured in protests that began when Nkurunziza announced in late April that he would stand again, after Burundi's constitutional court gave him the green light.
The president survived a coup attempt last month and has since faced down international pressure, including aid cuts, aimed at forcing him to reconsider.
The opposition has rejected proposals for the new election timetable, saying conditions for holding fair polls were not met.
"If things remain as they are, we consider that it will be a masquerade, a parody of elections," Charles Nditije, an opposition leader, told the AFP news agency late on Monday, demanding the setting up of a new independent electoral commission.
The government said the electoral commission's proposal to delay the presidential election until July 15 was the final time polls would be postponed.
The electoral commission has also suggested delaying parliamentary elections until June 26, and senator elections to July 24.
The parliamentary election had been scheduled to take place on June 5 but was postponed indefinitely on the eve of the vote, while the presidential vote was initially scheduled for June 26.
Meanwhile, the UN human rights chief on Tuesday warned that increasing violence by a pro-government militia, including executions, abductions and torture, was threatening to destabilise further the crisis-wracked central African nation.
"They could tip an already extremely tense situation over the edge," Zeid Ra'ad Al Hussein said in a statement.
"The last thing Burundi needs after a decade of gradual and largely successful peace-building is to be catapulted back into civil war because of a small number of people's ruthless determination to retain, or gain, power at any cost."
Almost 100,000 Burundians have sought refuge in neighbouring countries.
But Nzobonariba said the "vast majority" had "fled because of the terrorist rumours spread by politicians who do not want elections, aided by some foreigners and some non-governmental organisations who want to push Burundi into chaos."