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.

In the last 48 hours, two U.N. peacekeepers from South Africa and one from Uruguay were killed and 11 peacekeepers were injured and hospitalized.

The latest fighting has intensified a humanitarian crisis, displacing more than 400,000 people since the start of the year, and raised fears of sparking a regional war, the U.N. has warned.