Burundi is withdrawing its forces from eastern Democratic Republic of Congo where they had been fighting against M23 rebels, four sources said on Tuesday, in a further blow to Congo's army as it struggles to halt a rebel advance.

Rwanda-backed rebels appeared to be heading toward a third major city in eastern Congo, frightened residents said Tuesday, as international pressure rose over the M23's expansion in the mineral-rich region that's critical for global technology.

M23 rebels in the Democratic Republic of Congo have captured the eastern city of Bukavu with the backing of Rwandan forces, the second major city in the mineral-rich Kivu region to fall in three weeks.