Machar said the cease-fire announced by the government Friday needed to be backed up by guarantees of its legitimacy before he would halt the fighting and take part in peace negotiations.
"When one says there is a unilateral cease-fire, there is no way that the other person would be confident that this is a commitment," Machar told the BBC by telephone.
The government of President Salva Kiir has been locked in a power struggle with Machar, his former vice president, whom Kiir accused of plotting a coup. Kiir belongs to the ethnic Dinka group and Machar is from the Nuer community, the BBC said.
The Saturday march on Bor, the capital of Jonglei state where government troops wrested control from rebels last week, mostly involved members of an ethnic Nuer militia known as the White Army, the BBC said.
Kiir Friday released two Machar allies. U.S. Envoy Donald Booth told the BBC he hoped the newly released detainees would use their influence with Machar to "participate in a constructive manner in the efforts to bring about peace ... and resolve the political issues that sparked this conflict."
The fighting, which has driven an estimated 121,000 people from their homes, persisted this weekend. Clashes were reported in the oil town of Malakal, and the United Nations deployed reinforcements to help secure refugee camps.