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 rebel group has made significant territorial gains along the border with Rwanda in recent weeks, closing in on Goma, the provincial capital.

Rwanda-backed rebels entered the outskirts of eastern Congo’s largest city, Goma, on Sunday, causing what the United Nations called “mass panic” among its 2 million people and leading