The Hutu National Liberation Forces (FNL) is the only group still outside a power-sharing agreement aimed at ending a 13-year civil war.
Burundi has long been dominated by its Tutsi minority but now has a Hutu leader - ex-rebel Pierre Nkurunziza.
A ceasefire signed by the FNL last year soon broke down. But correspondents say both sides now seem committed to peace.
Opening the talks, Tanzanian President Jakaya Kikwete told both sides they could ruin the country by letting the "guns usurp the place of reason" or they could agree to negotiate a peace deal.
South African Security Minister Charles Ngqakula is to mediate at the talks in Dar es Salaam.
The BBC's Prime Ndikumagenge in Burundi says the FNL has imposed different conditions from the other rebel groups, maintaining that the problem in Burundi is between Hutus and Tutsis and demanding direct negotiations with Tutsi army or civilian leaders.
The current round of talks had been delayed by a disagreement over who would broker the talks, but both sides have now agreed to Mr Ngqakula's mediation.
However, Reuters news agency reports that the two sides have not yet agreed on what will be discussed.
Some 300,000 people were killed in the civil war, which was sparked in 1993 by the assassination of Burundi's first Hutu head of state and democratically elected president, Melchior Ndadaye.
Fighting between the government and FNL resumed in May 2005, only one week after the two sides signed an agreement.
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