The closed-door talks, mediated by regional power Uganda, began earlier in the day but quickly descended into an acrimonious exchange with no sign of any consensus on how to end months of turmoil in the central African nation.
Presidential elections are due to take place in the impoverished country on Tuesday, and the opposition looks poised to boycott the polls altogether -- leaving Nkurunziza headed for certain victory but the country violently divided.
Opposition groups say another term would also violate a peace deal that paved the way to end a dozen years of civil war in 2006.
During today's talks, sources said the government side accused the opposition of siding with the coup plotters, who in recent weeks have revived their attempt to overthrow the president by launching armed rebel operations in the north.
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The opposition in turn accused the government of being unwilling to negotiate.
Burundi's main opposition leader, Agathon Rwasa, also accused the government of using "delaying tactics just to stay in power."
Three out of the eight presidential candidates have already pulled out of the race, according to an open letter addressed to Burundi's election commission.