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 Congo’s government to call it a “declaration of war.

Rebels in Democratic Republic of Congo are closing in on a key city in the mineral-rich region aided by neighboring Rwanda’s army, the head of a peacekeeping mission told United Nations Security Council members during an emergency meeting on Sunday.

Congo has severed diplomatic ties with Rwanda as fighting between Rwanda-backed rebels and government forces rages around the key eastern city of Goma, leaving at least 13 peacekeepers and foreign soldiers dead and displacing thousands of civilians.