Rwanda-backed rebels claimed on Monday they captured eastern Congo’s strategic city of Goma, the hub of a region containing trillions of dollars in mineral wealth that remains largely untapped.

M23 rebels in the Democratic Republic of Congo were reported to have taken the key eastern city of Goma in fierce fighting with government forces that sent streams of refugees fleeing area communities.

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.