Rwanda-backed rebels who have captured eastern Congo’s largest city say they plan to take their rebellion to the capital and seek to gain political power.

The United Nations expressed deep concern at reports of M23 rebels and Rwandan troops advancing south towards the Congolese city of Bukavu on Thursday, as the militants sought also to assert their control over east Congo's largest city Goma.

President Paul Kagame said Rwanda was ready for "confrontation" as he rejected criticism over his backing for M23 rebels who were pushing south on Thursday in eastern Democratic Republic of Congo after capturing the major city of Goma.