Rwandan-backed rebels marched into east Congo's largest city Goma on Monday and Congolese troops exchanged fire with the Rwandan military across the border, in the worst escalation of a long-running conflict for more than a decade.

Residents of Goma reported gunfire and shelling after rebels overran Congolese troops. U.N. officials estimated that more than 1 million displaced people were now inside the city.

Ethnic Tutsi-led M23 rebels, backed by Rwanda, have infiltrated Goma in the Democratic Republic of Congo, escalating tensions and renewing fears of regional war. The conflict stems from the fallout of Rwanda's 1994 genocide,