Burundi's transitional President Domitien Ndayizeye has been warned against changing the draft constitution to allow himself to run in elections.
A referendum on the constitution has been delayed
The chief mediator in Burundi's peace talks, South African deputy President Jacob Zuma, said the move could damage efforts to end the 10-year war.
About 5,000 United Nations peacekeepers are in the country to support the South African-brokered peace process.
President Ndayizeye's supporters staged a protest earlier this week.
They were demanding the constitution be amended to allow him to contest presidential elections due to be held later this year.
Mr Zuma has just concluded two days of talks with Burundian leaders.
Meanwhile, the only rebel movement still active in Burundi, the National Liberation Forces of Agathon Rwasa, says it is ready to hold talks with the government.
FNL spokesman Pasteur Habimana told local radios in Bujumbura that international efforts were under way to boost the talks.
The Burundian government ended talks with the FNL in August last year after it claimed responsibility for a massacre of refugees.
A presidential spokesman said the government would set up a commission to pursue the negotiation process.