Machar previously demanded the release of 11 detainees accused of conspiring in an alleged coup -- which Machar denies existed -- before negotiations, the BBC reported.
In an interview Monday with the BBC, South Sudan President Salva Kiir, who said Machar was behind a coup attempt, ruled out any power sharing with his former deputy to end violence that has killed at least 1,000 people in the last two weeks.
"These men have rebelled. If you want power, you don't rebel so that you are rewarded with the power. You go through the process," Kiir said.
Kiir also has refused to release Machar's political allies who were arrested when the president voice the coup plot allegations.
The BBC said it hasn't been confirmed that Bor is under control of Machar's forces, a mix of soldiers loyal to him and an ethnic militia called the "white army."
A U.N. spokesman said Bor came under attack at daybreak Tuesday. Thousands of residents have fled Bor in recent days.
A South Sudanese army spokesman confirmed a "big fight" had happened in Bor. Machar later told the BBC he dispatched delegation to Addis Ababa, Ethiopia, for peace talks.
Uganda's president threatened military action against the rebels if they didn't agree to a cease-fire by the end of Tuesday and enter into negotiations.