GOMA, Democratic Republic of Congo (Reuters) -Rwandan-backed rebels marched into east Congo's largest city Goma on Monday and troops from the east African neighbours exchanged fire over the border in the worst escalation of the long-running conflict for more than a decade.

In the aftermath of its withdrawal from Goma, the M23 suffered a series of heavy defeats at the hands of the Congolese army, which was backed by a multinational force. The M23 were subsequently expelled from the country. M23 fighters then agreed to be re-integrated into the army in return for promises that Tutsis would be protected.

Congo has severed diplomatic ties with Rwanda as fighting between Rwanda-backed rebels and government forces rages around the key eastern city of Goma.

Congolese rebels said on Sunday they had taken Goma, the biggest city in the east of the mineral-rich country, after a lightning advance that has forced thousands of people to flee and fueled concerns of a regional war.