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.

The M23 militia, funded and directed by Rwanda, said it had seized the city of Goma, terrifying its people, many of whom sought shelter there after fleeing the rebel advance.