Forces from the Seleka rebel alliance have entered the town of Sibut after the army withdrew on Friday evening.
Meanwhile government officials have confirmed that their forces’ attempt to retake Bambari on Friday had been beaten back.
Rebel forces are now within 150km (95 miles) of the capital, Bangui.
President Francois Bozize has appealed to France, the United States and neighbouring countries for help to combat Seleka - an alliance of three separate groups who want the terms of an earlier peace deal to be honoured.