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 U.N. said the rebels were supported by at least some regular Rwandan troops, in the worst escalation of a long-running conflict for more than a decade.

The rebel militia M23, backed by Rwanda, captured Goma, a major city in eastern Democratic Republic of Congo, on January 27, marking a significant escalation in the conflict. M23 had briefly seized the city in 2012 but had been dormant for years.