The U.N. says Rwanda-backed rebels are expanding their presence in eastern Congo after capturing the region's major city.

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.

Rwanda-backed M23 rebels have expanded their presence in eastern Congo, capturing Goma, the region's major city. The UN expressed concerns over recent executions by the rebels following their intensified rebellion.