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.

Congolese security forces on Tuesday tried to slow the advance of Rwanda-backed M23 rebels who say they have captured Goma after entering eastern Congo's largest city, as U.N. officials reported violence,