Forces loyal to Ivory Coast's UN-backed president-elect Alassane Ouattara say they have launched an offensive aimed at sealing the border with Liberia.
They have swept down from their northern powerbase and claim to have taken the western town of Duekoue.
Officials from the army, which still backs rival leader Laurent Gbagbo, have confirmed that pro-Ouattara forces are trying to advance down to the western coast, but deny that they have taken Duekoue.
The pro-Ouattara fighters have also begun fighting in two other areas, the BBC reports. Known as the New Forces, they already control the north of the country and have taken four towns in the west.
If confirmed, Duekoue would be the fifth and biggest town to fall so far.
Pro-Gbagbo troops have lost every battle with the New Forces since last November's election, the BBC's correspondent in the central town of Bouake reports.
Mr Gbagbo refuses to step down as president despite international observers saying Mr Ouattara won the election last November.