Rwanda-backed rebels were quickly 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 following a major escalation of their yearslong rebellion.

There has been fierce fighting as Rwanda-backed M23 rebels advance towards the second-largest city in eastern Democratic Republic of Congo, marking a further escalation in the region's conflict. M23 fighters have been moving south, towards Bukavu, the capital of South Kivu, after capturing the largest city in the region, Goma.

Rwandan-backed M23 rebels asserted their control over east Congo's largest city Goma on Thursday by calling on residents to resume normal life, even as the group clashed with Congolese troops as they tried to take more territory.