Rwanda-backed rebels claimed they captured eastern Congo’s largest city, Goma, early Monday, as the United Nations described a mass panic among its 2 million people and Congo’s government said the rebel advance was a “declaration of war.

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.

Heavy artillery fire could be heard in Goma on Monday, hours after Rwandan troops and fighters from the M23 rebel group entered the capital of North Kivu Province. The UN has warned of a looming crisis.