Rebels in the Central African Republic (CAR) say they have captured two towns, as they vow to resume an offensive to oust President Francois Bozize.
Seleka fighters had taken the northern towns of Bouca and Batangafo, their military operations chief Arda Akouman told BBC Afrique.
A government spokesman said the towns were under threat, but had not fallen.
Seleka accuses Mr Bozize of failing to honour a peace deal signed in January - a charge he denies.
On Wednesday, the UN Security Council said it was concerned about a renewed conflict that could "jeopardize the precarious stability" of CAR.
Seleka joined a power-sharing government in January, in talks brokered by regional leaders, to end the rebellion they launched last year.
Under the deal, senior Seleka leader Michel Djotodia became the defence minister.
Foreign troops, including those from South Africa, were deployed to help enforce the peace deal.
On Sunday, Seleka said it had recalled Mr Djotodia and four other ministers from the government, and demanded the withdrawal of South African troops protecting Bangui.
Seleka said it would consider its options if the group's demands were not met by Wednesday.
As the deadline expired, Mr Bozize issued a decree to lift a curfew in the capital, Bangui, and to release political prisoners.
Mr Akouman told BBC Afrique that Seleka "don't believe what Bozize is saying" and the group's fighters had seized Bouka and Batangafo, about 300km (190 miles) and 600km respectively from the capital.
"We'll be going towards Bangui on foot or in pick-up trucks," he said.
"We don't want to have a dialogue with Bozize any more."
Government spokesman Gaston Mackouzangba denied the two towns had fallen, Radio France International (RFI) reports.
The towns were under threat, but still under the control of the army and peacekeepers, he said.
South Africa's foreign ministry spokesman Clayson Monyela rejected rebel demands for the withdrawal of his country's troops, Reuters news agency reports.
"The troops are staying. They aren't going anywhere," he is quoted as saying.
Under the peace deal, Seleka agreed that Mr Bozize, who seized power in a coup in 2003, would remain president until elections in 2016.
CAR has been hit by a series of rebellions since independence from France in 1960.
It is one of the poorest countries in Africa, despite its considerable mineral resources.