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.

The US is feeling pressure to intervene after its embassy in Kinshasa was attacked on Tuesday by anti-M23 offensive protesters in eastern DRC.