Rebels in the Central African Republic have repeated their demands that any peace deal must include President François Bozizé stepping down.
Peace talks between the Séléka Alliance and Bozizé to end the month-long crisis are scheduled to start on Tuesday in Gabon's capital, Libreville, under the auspices of the central African regional bloc CEEAC.
Eric Massi, a Paris-based spokesman for the rebels, made the demand, saying the insurgents were hoping to achieve a political solution that would restore peace but that "Bozize's departure is non-negotiable".
But Bozizé, who came to power in a coup in 2003, has flatly refused to go.
Rebels leaders on the ground in the Central African Republic (CAR) meanwhile said that their flight to Gabon set for Sunday was postponed to Monday.
But they insisted they would take part in the talks, which have the support of the UN Security Council and the United States.
"We are ready. We want to leave for the negotiations," Colonel Djouma Narkoyo told the news agency AFP.
Bozize's representatives were also due in Gabon on Monday, while the president himself plans to travel there only once the talks have officially begun, a source close to the presidency said.
The Seleka rebel coalition, which says Bozize has not abided by terms of earlier peace deals, launched an offensive on December 10 in the north and easily overran an ill-equipped and poorly trained army.
They marched across a large part of the former French colony, capturing key towns along the way, before halting their push within striking distance of the capital Bangui, in the southwest.
The rebels at the weekend captured two more towns, officials said, but there was no immediate reaction from the rebels themselves to the claim.
Unrest in the landlocked equatorial country has alarmed the country's neighbours and the international community, including the UN Security Council.
"The Security Council reiterated their demand that the Séléka coalition of armed groups cease all hostilities, withdraw from seized cities, and cease attempts to advance further," the 15-nation body said in a statement released on Friday.
Central African nations have begun sending reinforcements to Damara, the last major town between the rebels and the capital, to bolster the army against the rebels.
The regional troops are fighting under the banner of the multinational African force FOMAC, which CEEAC launched in 2008 in a bid to stabilise the country.
Northern neighbour Chad, whose President Idriss Deby is an ally of Bozize, has contributed most of the troops to the force, which is due to reach its full strength of 760 by the end of the week.
South Africa has authorised the deployment of 400 soldiers to the country, President Jacob Zuma's office said on Sunday.