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 took the city of Goma, terrifying its people, many of whom sought shelter there after fleeing the rebel advance.

The three-year insurgency by the Rwanda-backed rebel group M23 has intensified, and the U.N. warned the violence could spill into a wider war.