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.

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.

The three-year insurgency by the Rwanda-backed rebel group M23 has intensified, and the U.N. warned the violence could spill into a wider war.