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 rebels announced the capture of Goma in a statement minutes before a 48-hour deadline imposed by the group for the Congolese army to surrender their weapons expired.

The M23 rebel group has made significant territorial gains along the border with Rwanda in recent weeks, closing in on Goma, the provincial capital.