Congolese rebels and allied Rwandan troops claimed on Monday to have captured the mineral-rich city of Goma, as thousands fled fighting that killed at least 13 peacekeepers over the weekend.

M23 rebels in the Democratic Republic of Congo were reported to have taken the key eastern city of Goma in fierce fighting with government forces that sent streams of refugees fleeing area communities.

The Democratic Republic of Congo’s government says the Rwanda-backed rebels’ advance constitutes a “declaration of war."

Footage shows troops, some in army uniform, handing over arms to Rwandan security officers after crossing into eastern neighboring country at the main border crossing in Rwanda's Rubavu district - Ana

Heavy artillery fire could be heard in Goma on Monday, hours after Rwandan troops and fighters from the M23 rebel group entered the capital of North Kivu Province. The UN has warned of a looming crisis.

M23 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.

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.