Columns of M23 fighters allied with Rwandan troops on Sunday entered the centre of another key city in the eastern Democratic Republic of Congo as the African Union highlighted growing fears that the strife-torn country could break up.

Rebel forces have been seen in the center of Bukavu, the second largest city in the eastern Democratic Republic of Congo (DRC), on Sunday, residents told CNN, as the fighters push to expand their foothold in the mineral-rich region.

Congolese United Nations ambassador Zenon Mukongo Ngay submitted the letter to the Security Council denouncing its inability to rally in support of the DRC following the attack on the capital of South Kivu province in the country's east.