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.

Rebels seized the airport of east Congo's largest city, Goma, on Tuesday, potentially cutting off the main route for aid to reach hundreds of thousands of displaced people, after capturing the city in an offensive that left dead bodies lying in the streets.

Rebels backed by Rwanda captured more towns in eastern Congo as fighters moved beyond the key city of Goma in an apparent attempt to expand their control in the conflict-battered region