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.

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.