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.

M23 insurgency has intensified, with the Rwanda-backed rebels seizing control of more territory than ever before, prompting the U.N. to warn of the risk of a broader war. South Africa said nine of its peacekeepers were killed in fighting.

Congo has severed diplomatic ties with Rwanda amid intensifying M23 insurgency and ongoing battles. Allegations of Rwanda's involvement in the conflict have been denied by Rwanda, but tensions rose with the killing of a Congolese military governor.