Rwandan-backed rebels moved to tighten their grip on the strategic city of Goma in the east of the Democratic Republic of Congo on Monday in what Kinshasa described as “a declaration of war” by its neighbour.

Residents of Goma reported gunfire and shelling after rebels overran Congolese troops. U.N. officials estimated that more than 1 million displaced people were now inside the city.

Residents in eastern Congo’s largest city of Goma are fleeing after Rwanda-backed rebels claimed to have captured the regional hub from Congolese forces.