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.

Rebels in Democratic Republic of Congo are closing in on a key city in the mineral-rich region aided by neighboring Rwanda’s army, the head of a peacekeeping mission told United Nations Security Council members during an emergency meeting on Sunday.