Democratic Republic of Congo President Felix Tshisekedi has vowed “a vigorous and coordinated response” against a rebel alliance that has besieged swaths of the nation’s mineral-rich east and forced hundreds of local troops and foreign mercenaries to surrender.

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.

Democratic Republic of Congo President Felix Tshisekedi, speaking publicly for the first time since Rwandan-backed rebels seized the eastern city of Goma, said his government was preparing a response and warned of “unpredictable consequences” for the region.

U.S. Secretary of State Marco Rubio called for a urgent cease-fire in the conflict in eastern Democratic Republic of Congo saying Washington was "deeply troubled" by a recent escalation in the fighting.

Rwanda, which diplomats say backs M23 fighters who seized Goma in Democratic Republic of Congo this week, called on Wednesday for a ceasefire across eastern Congo and for Congo to negotiate with the rebels while denying Rwandan troops were involved.

Angolan President Joao Lourenco, who’s been mediating peace talks between Rwanda and Democratic Republic of Congo, called for the withdrawal of Rwandan-backed M23 rebels from the city of Goma and for Rwandan troops to leave Congolese territory.