UN Secretary General Ban Ki-moon has warned African leaders not to let the conflict in the eastern Democratic Republic of Congo engulf the region.
Speaking at an emergency summit in Nairobi, Mr Ban said the leaders had a historic responsibility to take action to help bring the fighting to an end.
Meanwhile unnamed UN officials said Angolan troops were fighting alongside the Congolese government army.
New clashes between government and rebel forces have broken out near Goma.
Thousands of displaced people have fled in panic from a nearby camp, as the clashes continue.
See detailed map of the area
Rebels have been fighting government troops in eastern Congo since August.
DR Congo has accused UN peacekeepers of failing to stop rebels from killing civilians in the east of the country.
Mr Ban is meeting DR Congo leader Joseph Kabila and Rwandan leader Paul Kagame, in the hope of reviving a peace deal signed in January.
But he said a recent offensive by Gen Laurent Nkunda's CNDP rebels had radically compounded the situation and led to a humanitarian crisis.
"The recent military offensives by the CNDP have radically compounded the situation, led to severe humanitarian consequences and thrust the eastern DRC once more into a phase of heightened crisis. This crisis could engulf the broader sub-region," Mr Ban said, quoted by AFP news agency.
"As leaders of Africa, you have a historic responsibility, it is a critical moment for the Great Lakes region, and for Africa as a whole. We must put the cycle of violence behind us."
The BBC's Mark Doyle in Nairobi says Mr Ban's words are diplomatic code telling Rwanda to stop supporting Gen Nkunda.
However, in a separate development a Uruguayan officer serving with the UN peacekeepers in DR Congo said the government side was being reinforced by Angolan troops.
The officer spoke to international news agencies in Goma, saying the Angolans had arrived there four days ago.
Angola is an ally of the Congolese government and has been invited by Mr Kabila to provide military assistance. But its government has said it will not intervene directly.
Angola and Zimbabwe both supported DR Congo with troops during the 1998-2003 war.
Gen Nkunda is not attending the talks, but he is in contact with the UN and other delegates at the summit.
UN peacekeepers said gunfire erupted on Friday just north of the city of Goma, where rebel forces halted their offensive last week.
Thousands of refugees in the nearby town of Kibati are said to have fled towards Goma when the mortar and small arms fire broke out.