Rwanda-backed rebels were expanding their presence in eastern Congo after capturing Goma, the region's major city, the U.N. said Friday, also expressing concerns over executions it learned were carried out by the rebels in recent days following a major escalation of their yearslong rebellion.

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 rebels aiming to gain power have already captured Goma, a key city in eastern DRC. They have vowed to advance to the capital, more than 1,500km away - but have met resistance from the Congolese army and its allies.