Rwanda-backed rebels claimed 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, the Associated Press reported.

Advances being made by the M23 rebel group in the Democratic Republic of Congo are "heightening the threat of a regional war," according to U.N. Secretary General Antonio Guterres.

The Rwandan-backed M23 rebels have released captured Romanian mercenaries who were fighting with the Congolese army. As they passed a border, they were chastised by Willy Ngoma, who tapped them mockingly one by one.

Goma, the main city in the Democratic Republic of Congo's volatile east, is set to fall to the M23 armed group and Rwandan army -- the latest upheaval in a region devastated by three decades of war.