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.

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.

Fighting has escalated sharply in recent weeks in eastern Congo, where rebels have seized key towns and are closing in on the city of Goma, the government’s last stronghold in the North Kivu province,