Ministers from Burundi's leading Tutsi parties have boycotted a cabinet meeting called to discuss the drafting of a new constitution.
President Ndayizeye must organise elections by end of October
Last month, the Uprona party and its nine allies refused to sign a South African-brokered power-sharing deal, to form the basis of the constitution.
They want further dialogue about power-sharing between Hutus and Tutsis, who make up 15% of the population.
Without a new constitution, elections due by 31 October cannot go ahead.
Political process jeopardised
The boycotters, led by Vice-President Alfons Marie Kadege, said they were not withdrawing from government.
But they insisted there needed to be a "national consensus" on power-sharing or the whole political peace process would be jeopardised.
60% Hutu, 40% Tutsis
3 Twa seats
50% Hutu, 50% Tutsis
3 Twa seats
Reacting to the boycott, presidential spokesman Panrace Cimpaye said as ministers in government they are beholden to President Domitien Ndayizeye, not their individual parties.
The cabinet meeting, he stressed, was to discuss a new constitution, not impose it, and as such was the dialogue the Tutsi parties were calling for.
The Pretoria accords state the national assembly and government comprise 60% ethnic Hutus and 40% Tutsis.
But Uprona wants seats to be given to parties as well as ethnic groups.
The Burundi parliament approved on Tuesday an independent electoral commission which includes three Hutus and two Tutsis to oversee the country's first elections.
Some 300,000 people have been killed since the civil war broke out in 1993.
About 5,000 United Nations peacekeepers are in the country to support the South African-brokered peace process.